Sir Keir Starmer's plan to boost defence spending by 2.5% and make Trident the "bedrock" of his national defence strategy has been branded "grotesque" by the SNP.

The Labour leader, who is expected to be elected Prime Minister in a general election later this year, paid a visit to Barrow where four nuclear submarines are being built.

Built at a cost of £7.75bn per unit, four Dreadnought submarines will replace the current Vanguard ships, based at Faslane on the west coast of Scotland, which constantly patrol the seas carrying ballistic missiles equipped with thermonuclear warheads.

The Trident system is designed to act as a deterrent to other nations with nuclear capability, as a first strike on the UK would not eliminate the ability to strike back given some of the country's weapons are continuously at sea.

Replacement of the current submarines was approved by the House of Commons in 2016 despite opposition from anti-nuclear groups and campaigners concerned about the cost.

In an interview with the i newspaper, Mr Starmer confirmed his ambition was to boost the defence budget to 2.5% of GDP, if it fits with Labour’s fiscal rules.

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He said: "The changed Labour Party I lead knows that our nation’s defence must always come first. Labour’s commitment to our nuclear deterrent is total.

“In the face of rising global threats and growing Russian aggression, the UK’s nuclear deterrent is the bedrock of Labour’s plan to keep Britain safe. It will ensure vital protection for the UK and our NATO allies in the years ahead, as well as supporting thousands of high paying jobs across the UK.

“Countless families in Barrow and across Britain have built a secure future over decades of hard work building our defences. I want that to continue for the decades to come.

“That’s why we are fully backing Aukus submarines to be built in Barrow, too. And it’s why Labour will ensure that new UK leadership within Aukus helps make this national endeavour a success for Britain.”

SNP defence spokesperson Martin Docherty-Hughes MP said: “Westminster has already wasted billions of pounds of taxpayers’ money on nuclear weapons and expensive nuclear energy.

The Herald:

“It is therefore grotesque that Sir Keir Starmer is prepared to throw billions more down the drain when his party claim there is no money to improve our NHS, help families with the cost of living or to properly invest in our green energy future.

“This money would be better spent on a raft of other things – not least investing in the green energy gold rush, which would ensure Scotland, with all its renewable energy potential, could be a green energy powerhouse of the 21st century.

“And while the UK Government wastes millions misfiring Trident missiles at the Defence Secretary, the urgent priority is more money for conventional defence and for our armed forces, who are underpaid and under-resourced.

“With both Rishi Sunak and Keir Starmer focused on the wrong priorities, it is only the SNP standing up for Scotland’s interests and Scotland’s values.”

The Herald: HMS Victorious, part of the Trident nuclear fleet

The concept of a nuclear deterrent is based on the doctrine of Mutual Assured Destruction (MAD), which holds that no country possession the weapons of mass destruction would use them against another nuclear-armed state because both would be destroyed the fighting.

Anti-nuclear campaigners maintain that the existence of the weapons is a danger to human life on the planet, and can point to a number of close calls since the USSR became the second country to enter the atomic age in 1949.

In 1961 a B-52 bomber carrying a nuclear weapon broke up in mid-air over North Carolina, with only a single switch preventing the bomb from detonating.

In 1983, Stanislav Petrov of the Soviet Air Defences received a warning that six missiles had been launched at the USSR by the United States but he accurately deduced that a malfunction had caused a false alarm and, against protocol, did not inform Moscow and thus prevented a potential retaliatory strike.

Kate Hudson, general secretary of the Campaign for Nuclear Disarmament, said voters are “desperately looking for hope from the Labour Party.

“However, it’s increasingly clear that Starmer’s offer is just more of the same: billions of pounds wasted on nuclear weapons and nuclear power, and a belligerent foreign policy that includes support for the Aukus pact, Nato, and continuing arms sales to Israel, used to kill Palestinians.

“Putting billions of pounds into the pockets of arms companies and their investors will not reinvigorate the economy in any meaningful way.”