The Conservatives’ manifesto will have a commitment to build four new nuclear missile-armed submarines, Defence Secretary Michael Fallon has said.
He accused Labour of using the nuclear deterrent as a “bargaining chip” with the SNP, which would vote to scrap it.
But Labour’s Chris Leslie called his comments “ridiculous” and said Labour was “totally committed” to a continuous, at-sea nuclear deterrent.
He said his party “would not do deals” on defence with the SNP or anyone else.
The Conservatives and Labour are both committed to replacing the UK’s ageing fleet of Vanguard class submarines which carry Trident nuclear missiles and maintaining the continuous, at-sea deterrent – meaning there is always one nuclear-armed vessel on patrol.
Labour has floated the possibility of reducing the number of submarines from four to three if a continuous deterrent could be maintained, though Mr Leslie told the BBC the party’s “current view” was that four were needed.
The Liberal Democrats favour cutting to three, saying the existing system was designed for the Cold War era.
Policy guide: Foreign and defence
This election issue includes foreign policy and the role of UK’s defence forces at home and abroad.
The Clyde-based submarines that currently carry Trident are due to reach the end of their operational lives within the next decade, and a final decision on replacing the system is due in 2016.
The government in 2013 put the bill at between £15bn and £20bn. However, Greenpeace argued it will run to at least £34bn once extra costs such as VAT are taken into account.
The Lib Dems have said ordering fewer submarines would save up to £4bn in the long-term, though that claim is disputed.
The SNP has said it would work with Labour in order to keep the Conservatives out of government after the 7 May general election, but it opposes renewing Trident and calls the issue a “red line”.
Labour has said it will not form a coalition with the SNP, but Tory leader David Cameron has said Labour failed to rule out a post-election “deal”.
In an article for the Times, Mr Fallon confirmed the existing Tory policy of four new submarines would feature in the election manifesto.
He also said Ed Miliband was “a man so desperate for power he is ready to barter away our nuclear deterrent in a backroom deal with the SNP”.
SNP leader Nicola Sturgeon’s “brusque demands should alarm people right across our United Kingdom”, Mr Fallon said.
“Our nuclear submarines protect all of Britain including Scotland. This SNP policy is a threat to us all that would dangerously weaken our collective defence,” he added.
But Mr Leslie said Mr Fallon’s claims were “totally fabricated”, adding: “What we’re going to get now, it seems, in the election campaign is the Conservatives basically lying and smearing us on these issues with no evidence whatsoever.”
He said the Conservatives were “desperate to change the headlines” after Labour’s pledge to scrap non-dom tax rules on Wednesday and were “stooping to a low level”.
Mr Leslie said Labour would maintain Britain’s “continuous, at-sea nuclear deterrent”.
“It’s important for the defence of our country and our place in the world,” he said.
Speaking at Wednesday night’s Scottish political leaders’ debate on BBC One Scotland, Ms Sturgeon said: “It is often asked of me: is Trident a red line? Well here’s your answer – you’d better believe it’s a red line.”
She added: “There is no circumstances under which SNP MPs will vote for the renewal of Trident.”
But she did not say the SNP would bring down a minority government if it lost a vote on Trident.
The issue provoked a coalition row in the previous government when the Lib Dems requested a report examining possible alternatives.
Sir Nick Harvey, the Lib Dem defence spokesman, told Radio 4’s Today programme the nuclear deterrent was “worth retaining”.
But he said it did not need to be “patrolling the seas 24/7 when we have no known nuclear adversary”.
Maintaining three submarines would allow the UK to maintain “continuous at-sea deterrence”, he said. Two submarines would provide a “perfectly meaningful deterrent”, but not on a “continuous basis”, he added.
All of the parties have faced questions during the election campaign about their plans for defence spending.
Neither Labour nor the Conservatives have committed to matching the Nato target of 2% of GDP beyond 2016. UKIP says it would meet this commitment by cutting foreign aid.
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