The Prime Minister has warned that Jeremy Corbyn's opposition to maintaining the nuclear deterrent is "quite wrong" and the UK could not "relax our guard" by abandoning the Trident system.

Theresa May, in her first Commons speech since becoming the country's second woman Prime Minister, sought to highlight Labour's divisions over one of the key planks of the UK's defences.

Her comments came after Labour leader Mr Corbyn was warned by the head of the GMB union that he faces a backlash from trades unionists if he votes against the replacement of the ageing Vanguard submarines that carry the Trident missiles.

Mrs May told MPs the "very real" threat posed by Russia and North Korea meant that the UK should remain a nuclear power with a round-the-clock capability to launch a missile from a submarine.

In a swipe at Mr Corbyn, she said: "Some people suggest to us that we should actually be removing our nuclear deterrent. This has been a vital part of our national security and defence for nearly half-a-century now and it would be quite wrong for us to go down that particular path."

She highlighted Labour's manifesto commitment to maintaining the continuous-at-sea deterrent.

John Woodcock, the Labour MP for Barrow where the submarines will be build, told Mrs May that it remained "steadfastly" the party's policy to retain the deterrent and many Opposition MPs would "do the right thing", despite Mr Corbyn's unilateralist position.

The Prime Minister said: "The threats from countries like Russia and North Korea remain very real. As our Strategic Defence and Security Review (SDSR) made clear, there is a continuing risk of further proliferation of nuclear weapons.

"We must continually convince any potential aggressors that the benefits of an attack on Britain are far outweighed by their consequences.

"And we cannot afford to relax our guard or rule out further shifts which would put our country in grave danger. We need to be prepared to deter threats to our lives and our livelihoods and to those of generations who are yet to be born."

She insisted that the UK would maintain the "full spectrum of our defences at full strength", continuing to meet the Nato target of spending 2% of GDP on defence and going ahead with the projects outlined in the SDSR.

Labour is expected to split three ways on the issue. Mr Corbyn is expected to vote against the Commons motion, while other frontbenchers will abstain or support the Government.

The head of the GMB trade union Tim Roache insisted Mr Corbyn should abide by existing Labour Party policy, which is to back Trident renewal. He warned that 45,000 jobs around the country - many of them highly skilled - were dependent on the programme going ahead.

Mr Roache, who supported Mr Corbyn as leader, told BBC Radio 4's The World at One: "The Labour Party have a clear policy. The clear policy is that Labour will uphold an at-sea deterrent.

"I would expect therefore all Labour MPs, including the leader of the Labour Party - in fact, especially the leader of the Labour Party - to uphold that current policy."

With Mr Corbyn set to face a leadership challenge from either Angela Eagle or Owen Smith, Mr Roache said he would now be balloting his 640,000 members on whether they believed he was still the right person to lead the party.

Mr Roache's comments came as Labour's deputy leader Tom Watson warned of "consequences" for trade union bosses, such as Unite general secretary Len McCluskey, who continued to back Mr Corbyn.

He told The World at One: "You have Len McCluskey strongly supporting Jeremy Corbyn, who will be voting against the Trident programme tonight, which will put many defence workers in Unite out of their jobs if he gets his way.

"If I was a defence worker in Unite and I was reading social media that Unite were about to give Jeremy Corbyn a quarter of a million pounds of my subscriptions, I would be furious."

While Mr Watson said he expected the "vast majority" of Labour MPs to support the Government motion backing Trident renewal, he acknowledged that the continuing divisions were harming the party.

He said: "It is not a great position for the Labour Party to be in, to be honest. It doesn't show clarity of thinking, nor do I think it would reassure people that we are strong enough on security matters.

"We might know that this is political skulduggery by the Tories but the country expects us to know our position on strategic defence matters, particularly something so important as our independent nuclear deterrent."

Shadow foreign secretary Emily Thornberry - who, together with shadow defence secretary Clive Lewis, said she will abstain - insisted the vote was irrelevant and accused the Government of "playing games" to embarrass Labour.