Sir Keir Starmer has said he would be prepared to use nuclear weapons if needed to defend the UK as he set out Labour’s defence plans.

The Labour leader said “security will always come first” under his leadership and claimed his party has left behind Jeremy Corbyn’s opposition to the Trident nuclear weapons system.

If elected, Sir Keir said he would increase defence spending and update the UK’s nuclear arsenal.

Conservative defence secretary Grant Shapps said Labour represented a “danger to our national security”.

He called Labour’s pledge to protect the nuclear deterrent “meaningless”, given a dozen of Labour’s current front bench team voted against renewing Trident in 2016 under Mr Corbyn, including shadow foreign secretary David Lammy and deputy Labour leader Angela Rayner.

“Uncertain times call for a clear plan and bold action to chart a course to a secure future, only the Conservatives offer that,” he added.

In the 2016 vote, Mr Lammy cited his Christian faith when explaining why he voted against it.

The Labour party was split when the House of Commons last voted to renew the UK’s Trident nuclear weapons system, with 140 of the party’s 230 MPs defying leader Mr Corbyn to back the motion.

But Sir Keir – who did vote to renew Trident – claimed he had his “whole shadow cabinet behind me” on plans to maintain the nuclear deterrent.

“This is a changed Labour party and the most important thing is I voted in favour of a nuclear deterrent,” he said.

“I lead from the front, I’ve always lad from the front.”

Asked by BBC Political Editor Chris Mason, if he would authorise the firing of nuclear weapons if he was prime minister, Sir Keir said: “We have to be prepared to use it.

“Now, nobody who aspires to be prime minister would set out the circumstances in which it would be used. That would be irresponsible.

“But it is there as a part, a vital part, of our defence.”

Asked if she had changed her mind since 2016, Ms Rayner said she still believes in “multilateral” nuclear disarmament, arguing “we should be looking at disarmament of nuclear weapons but that has to be done in combination with other countries”.

Sir Keir used his speech to recommit Labour to boosting UK defence spending to 2.5% of GDP – without setting a deadline. Prime Minister Rishi Sunak has vowed to hit the target by 2030 if re-elected.

“Keeping our country safe is the bedrock of stability that the British people rightly expect from their government,” Sir Keir said.

He committed Labour to a “nuclear triple lock”: continuing to build four new nuclear submarines in Barrow-in-Furness, maintaining Britain’s at-sea deterrent, and delivering all future upgrades for submarine patrols.

The Trident system, based near the Firth of Clyde, includes four nuclear-powered Vanguard-class submarines, missiles and warheads.

Each submarine is designed to carry 16 Trident missiles, capable of delivering multiple warheads – but in recent years, they have carried eight missiles each, with a maximum of 40 warheads per boat.

The UK is already in the process of building four new nuclear submarines in Barrow in-Furness at a cost of £31bn over the lifetime of the programme. The country maintains a continuous at sea nuclear deterrent with its existing fleet.

The Conservatives have also commitment to continue this polices as well as delivering future upgrades.

SNP Spokesman Martin Docherty-Hughes said: “In the middle of a cost-of-living crisis, it is objectively wrong that Keir Starmer would funnel billions of pounds of public money into keeping weapons of mass destruction on our doorstep in Scotland, while families are still living in poverty after 14 years of Tory austerity, and our budget from the UK government keeps getting slashed.

“Nuclear weapons have no place in Scotland, and only a vote for the SNP in July will protect Scotland’s interest against the Labour and Tories – neither of whom will do what the people in Scotland want and scrap Trident nuclear weapons for good.”

In another break from Mr Corbyn’s leadership, Sir Keir used his speech to push for the UK to assume a “leading” role in Nato.

Sir Keir’s predecessor criticised Nato’s role and expansion, particularly in conflicts he found unjust – though did not push for the UK to leave.

These positions led to accusations from its opponents that Labour was weak on national security during Mr Corbyn’s tenure.​

Sir Keir also announced he would keep a dedicated veterans minister in his cabinet if Labour wins election. The standalone role was created under Boris Johnson but later dropped and then restored by Rishi Sunak.

Ahead of his speech, Sir Keir unveiled a “record” 14 ex-military candidates, which he said “is evidence of a changed Labour Party”.

In recent years, armed forces veterans have been rare among Labour’s MPs.

In the last parliament only two Labour MPs had military service – Dan Jarvis, who served 16 years in the Parachute Regiment, and Clive Lewis who reached the rank of Lieutenant in the Territorial Army.

The ex-military candidates announced by Labour include:

• Al Carns, a Royal Marines colonel who stepped down from the military last week
• Calvin Bailey, a former Royal Air Force commanding officer
• Neil Guild, who served in the British Army for more than six years and now works as a civil engineer
• Louise Jones, a former army intelligence officer.

Speaking on Monday, Ms Jones, the Labour candidate in North East Derbyshire, said: “Labour has national security at its core. Our commitment to Nato is unshakable.

“Only labour recognises the duty we have as a nation to our armed forces, only Labour has the leader we can fully trust with our national security and only Labour will put this country first.”