The case is "stronger than ever" to embrace the non-replacement of Trident, which offers "serious strategic and economic benefits", the SNP leader in Westminster has argued.

Angus Robertson told the Commons the Ministry of Defence (MoD) had the "wrong priorities", investing billions on nuclear weapons which it could never use, but not properly managing the conventional armed forces.

The SNP and Plaid Cymru called today's debate on an opposition day motion to highlight their commitment to scrapping the programme and instead using what they claim would be £100 billion saved to ease austerity measures.

Mr Robertson said it was the first opportunity to debate Trident replacement since the publication of the Government's update in December 2014.

This, he added, outlined a further £261 million had been re-profiled to be spent on the project ahead of the main gate stage "in early 2016" when MPs would take a decision on authorising construction of new submarines, "confirming that Trident is not subject to the Government's austerity agenda".

He said: "The time has come to put down a marker about scrapping Trident and not replacing these weapons of mass destruction."

At present, he said, the UK Trident submarine remained on patrol at all times and each submarine carried an estimated eight missiles each of which could carry up to five warheads, 40 in total.

Each warhead, he said, had an explosive power of up to 100 kilotons of conventional high explosive - eight times the power of the atomic bomb that was dropped on Hiroshima in 1945 which killed an estimated 240,000 people from blast and radiation.

He said: "I've yet to hear a supporter of Trident convincingly explain in what circumstances they are prepared to justify the killing of hundreds of thousands of innocent men, women and children and cause massive environmental damage to the world for generations to come.

"That is a consequence of the use of nuclear weapons and surely if one has them, one has to be prepared to use them. I've yet to hear anybody give an example of circumstances where they are prepared to kill millions and millions of people."

Mr Robertson said: "On the other hand, I think the case is stronger than ever to embrace the non-replacement of Trident which offers serious strategic and economic benefits."

These, he said, included improved national security through budgetary flexibility in the MoD and a more effective response to emerging security challenges in the 21st century, improved global security through a strengthening of the non-proliferation regime, deterring of nuclear proliferation and the de-escalation of international tensions.

The UK, he argued, would be lauded for making such a decision.

He said: "The point is that the MoD has the wrong priorities, investing billions on nuclear weapons which it can never use, but not properly managing the conventional armed forces which are so necessary.

"The National Security Strategy (NSS) noted in 2010 that in a period of changing security threats it would be sensible to consider how ending the Trident replacement programme would release resources that could be spent on more effective security measures."

Mr Robertson said that with the NSS placing international terrorism, cyber crime and major accidents and natural hazards such as coastal flooding at the top tier of threats to the UK, it suggested these areas needed greater resources "rather than the false priorities in terms of the nuclear deterrent".

On the cost of Trident replacement, he said studies had estimated they were "astronomic", approaching £100 billion, involving development, construction and in-service running costs over decades.

"It is billions and billions of pounds every year that could be saved and could be re-prioritised," he said, referring to the current debates on austerity and the growth of food banks.

Mr Robertson said the debate was an important opportunity to show that there was opposition to Trident renewal at Westminster and the election presented a chance to underline that by electing MPs with a policy of opposing Trident, including those from the SNP, Plaid Cymru and the Greens.

He said: "With polls showing we may very well hold the balance of power after the next general election we will do everything we can to ensure that Trident replacement does not go ahead."

Labour, he feared, would not represent the majority of its own supporters.

Responding, Defence Secretary Michael Fallon said: "Today's debate is about the primary responsibility of any government - the security of our nation, our freedoms and our way of life. It is not about short-term politics."

"Whatever the current threats to this country, what we cannot gamble with is tomorrow's security. That is why this Government and all previous governments for the last six decades have retained an operationally independent nuclear deterrent.

"Today this Government is committed to maintain that credible, continuous and effective minimum nuclear deterrent based upon Trident and operating in a continuously at-sea posture for as long as we need it."

Mr Fallon went on: "We can't gamble with our country's national security. We have to plan for a major direct nuclear threat to this country or to our Nato allies that might emerge over the 50 years that the next generation of our submarines will be in service.

"In a world that is getting more dangerous there are no alternatives that offer the level of protection and security that this country needs."

He raised the examples of Russia, North Korea and Iran and stressed the world could not simply "dis-invent" the some 17,000 nuclear weapons in circulation globally.

In addition, successive governments had repeatedly decided a full-time at-sea deterrent was required, he said. He also highlighted the 2013 review which found no alternative system to be as capable or as cost-effective as Trident.

And he insisted the Government only planned to replace its submarines in late 2020, subject to the main gate decision next year.

Mr Robertson intervened to ask what the total cost would be including through-life costs, suggesting it would be in the region of £100 billion.

Mr Fallon said he could not confirm this until after the main gate decision, but dismissed that figure. He also pointed out that the site at the Clyde was the largest employer in Scotland.

The Defence Secretary then criticised other parties' policies in turn.

He called the SNP's position not to renew Trident irresponsible and claimed it would sacrifice the security of the UK for "minuscule savings".

He also dismissed the idea of a "nuclear-free Scotland" as nonsense, saying an independent Scotland would currently rank seventh in the European league table due to the amount of electricity that is generated by nuclear power.

Mr Fallon then accused Labour leader Ed Miliband of going back on his party's commitment to maintaining the "minimum credible" nuclear deterrent continuous at sea because he was courting the SNP with a view to a coalition after May.

But shadow defence secretary Vernon Coaker confirmed Labour's position remained as before.

Finally, Mr Fallon criticised the Liberal Democrats, saying a part-time deterrent would not work. He also said sending out unarmed patrols was "pointless" and "absurd".

He concluded: "This country faces the threat of nuclear blackmail from rogue states. It is then contemptible for the SNP or the Liberal Democrats to suggest they might use the ultimate guarantor of our freedom and independence as some kind of bargaining chip in some grubby coalition deal.

"More simply put, it is only the Conservative Party that will not gamble with the security of the British people."

That was not mutually exclusive to a vision for disarmament, he argued, and he announced that the Government had met its 2010 Strategic Defence Review commitment to reduce the number of deployed warheads on each submarine from 48 to 40.

But unfortunately others had not followed suit, he added.

Labour MP Dame Joan Ruddock (Lewisham Deptford) said: "It is instructive to inquire how other countries and institutions do view us, the nuclear weapon states. I had an opportunity to find out about that when I attended a conference organised by the Austrian government about the humanitarian effects of nuclear weapons last December.

"Building onto previous meetings hosted by Norway and Mexico, this conference was attended by representatives of 157 governments. Most telling was the contribution of the international Red Cross and Red Crescent - the bodies on which the whole world depends, regardless of politics.

"Let me quote further to the statement they made - 'even though only a few states currently possess nuclear weapons they are a concern to all states. They can only bring us to a catastrophic and irreversible scenario that no one wishes and to which no one can respond in a meaningful way. All other weapons of mass destruction have been banned. Nuclear weapons, which have far worse consequences, must now be specifically prohibited and eliminated as a matter of urgency.'

"I don't think anything any of us could not respect a statement from the Red Cross and the Red Crescent.

"Around 40 countries have already signified their support for progressing towards finding an international treaty that could ban all nuclear weapons. Renewal of Trident flies in the face of such international action and must not be allowed to do so.

"The real threats to this country are cyber warfare, terrorism, climate change and pandemics. We need all the resources we can must to confront these threats and we cannot afford to squander billions of pounds on a weapon system that by general consent can never be used."

Tory Oliver Colvile (Plymouth Sutton & Devonport) said his constituency depended on submarines and backed the deterrent.

He said: "In my opinion it is the cornerstone of our membership not only of Nato but it is our security on the United Nations Security Council as well.

"Nobody knows what the outcome of May's general election is going to be but the SNP, the Greens, Plaid Cymru have all made it clear they won't enter coalition government with the Conservatives and according to the Independent on December 15, the SNP, Plaid Cymru and the Greens would demands their price for supporting the Labour Party in a hung Parliament would be a scrapping of the nuclear weapons programme.

"(The Liberal Democrat) approach would mean we would only have a part time deterrent. We would depend on a part time enemy.

"Scrapping or even reducing the number of nuclear submarines would have devastating impact upon my constituency.

"Retaining Britain's nuclear deterrent, a strategic concept that seeks to prevent war, is a key element and a cornerstone of the defence of our country."

Labour's Roger Godsiff (Birmingham Sparkbrook and Small Heath) said he had no ethical objection to nuclear weapons but would back the SNP motion backing a scrapping of the Trident renewal programme.

He said: "Nato has been the most successful mutual defence pact the world has ever seen. It never attacked anybody, unlike the Warsaw pact, it kept the peace of Europe for 50 years, and I was one of those that regretted the change in strategy of Nato to become the world's policeman.

"I think that was dangerous, I think it has put enormous strains on Nato, but still it is a very effective mutual defence pact.

"I argue that is where we get our security - not by having the mythical idea we have an independent nuclear deterrent."

SNP MP Angus MacNeil (Na h-Eileanan an Iar) said: "It seems to be the view of the UK Government... that any government not holding nuclear weapons is not taking defence and security seriously.

"The logical upshot of this, the Pyongyang policy or it's now maybe the London Tory policy, I think, is that everybody should have nuclear weapons.

"It's to me the global equivalent of the USA handgun policy and we know what trouble that has created in the society of the United States of America."

Labour's John Woodcock (Barrow and Furness) rejected suggestions that his party would scrap Trident to gain SNP and Plaid Cymru support in a future coalition government.

Mr Woodcock took aim at the parties for prioritising it as a "red line" in future coalition negotiations above issues such as health and the economy.

He told MPs: "It's for the nationalists to explain why they are seeking to prioritise unilateral disarmament by making it as far as I understand the one red line, the single red line that they would have in any future coalition talks, above all the pressing issues of health, jobs, education, the economy.

"All of those other pressing issues that matter to the people of Scotland and Wales.

"That is their one red line.

"And it is also for them to say how this stance fits with their desire to remain a member of Nato.

"But Labour's view is settled.

"The leader of the Opposition (Mr Miliband) will never accept an irresponsible deal that trades the nuclear security of future generations in a deeply uncertain world for nationalist support to enter Downing Street.

"We are going to carry on campaigning for a Labour majority.

"Plaid Cymru and the Scottish National Party can dream on."

Defence Select Committee chair Rory Stewart said the decision was far too important to be a question of economics.

He said: "Certain kinds of argument should no longer be relevant.

"This is a huge question, as pointed out by Labour MPs, this is a question of Armageddon.

"This is a question of deep, deep strategy.

"This is the fifth largest economy in the world, we should not be making a decision whether or not to keep nuclear weapons on the basis of either the belief that we could save some money by cutting them or alternatively the belief that we should be retaining them in order to keep some jobs in a marginal constituency, it is much more important than that."

The Tory MP also argued that it was unwise to change the nature of Britain's deterrent when the Government is unsure of the threat posed by Russia and president Vladimir Putin's aggressive stance in the world.

Mr Stewart said: "We don't know what Putin is doing and before we decide how to deter him we need to work out what that threat is.

"Is he intending to use nuclear weapons? We notice for example that he has been investing heavily in his tactical nuclear arsenal.

"He has also committed a great deal of money towards modernising his nuclear arsenal.

"He has been running recent exercises including the deployment of a nuclear bomber to Venezuela.

"At the same time the activities which he is engaging in, which his chief of staff is laying out, is all based around the idea of ambiguous warfare, almost at the very opposite end of the spectrum from nuclear war."

Mr Stewart added: "The fundamental rationale for all of this depends on something... (which is) the nub of the disagreement.

"Do we believe in a world order? Do we believe in Nato? Do we believe Britain is a global power?

"Do we wish to play a role in the world? If we do, I personally will be voting in favour of these weapons.

"But the deterrent will not make sense unless the character of the nation is in place."