Ed Miliband has said defence secretary Michael Fallon has "demeaned himself and demeaned his office" after the Tories were accused of an "embarrassing" personal attack on the Labour leader over the Trident nuclear deterrent, in the most bitter moment of the election campaign so far.
Writing in The Times this morning, Fallon said Miliband was willing to "barter away" Britain's nuclear deterrent by doing a deal with the SNP, because he was a "backstabber" who ran against his brother David to become Labour leader.
But speaking at a Labour education election launch, Miliband hit back strongly, saying: "Michael Fallon's a decent man but today he's demeaned himself and demeaned his office.
"Decent Conservatives across our country will say - come on, we're better than this. David Cameron should be ashamed.
"Why does a campaign descend into the gutter? Because a campaign is failing.
"They are desperate. Lynton Crosby is behind the scenes, pulling the strings, sending out minions. David Cameron should get a grip."
The Tories have accused Miliband of preparing to "stab the United Kingdom in the back" by trading away Britain's nuclear deterrent in order to secure power in a backroom general election deal with the Scottish Nationalists, who favour nuclear disarmament.
David Cameron is challenging Miliband to match the Conservative commitment to replace all four of Britain's nuclear submarines.
Writing in The Times ahead of his address, Mr Fallon said: "Ed Miliband stabbed his own brother in the back to become Labour leader. Now he is willing to stab the United Kingdom in the back to become prime minister."
Critics slammed the Tories for the "embarrassing" personal critique of the Labour leader, which some called a "smear" while Paul Waugh, the editor of Politics Home, said Labour may compare the dig to the Daily Mail's much-maligned attack on Miliband's father which called him "the man who hated Britain".
Fallon will confirm today that a new Conservative government will go ahead with the construction of four new Trident nuclear missile submarines to replace the existing fleet.
He is warning that Miliband's refusal to rule out a post-election deal with the SNP - which is committed to nuclear disarmament - would put the future of the deterrent in jeopardy.
The row broke out as Labour prepared to sign off its election manifesto at a "Clause 5" meeting of the shadow cabinet, together with senior officials from the ruling national executive, the national policy forum and the Parliamentary Labour Party.
Shadow defence secretary Vernon Coaker accused the Tories of "resorting to the language of smear" insisting the Labour leader had made "crystal clear" that Britain's national security was not a matter for negotiation.
Labour - like the Conservatives - is committed to replacing the ageing fleet of Vanguard class submarines which carry the Trident missiles and maintaining the continuous at-sea deterrent, ensuring there is always one nuclear-armed vessel on patrol.
However with the polls suggesting the SNP could be the third largest party in another hung parliament, Nicola Sturgeon has made clear that the price of their support after the election on May 7 would be scrapping Trident.
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Mr Fallon will say in his speech: "Nicola Sturgeon could not be clearer. She has told Ed Miliband that scrapping Trident - our country's vital nuclear deterrent - is a red line she will not cross.
"If he wants the keys to Number 10, he must abandon any plans to renew our current Vanguard ballistic missile submarines."
Miliband is accused of being a 'backstabber'
After Sturgeon used the first of the televised Scottish leaders' debates to offer to put Miliband in Downing Street, Fallon will say it is a "sign of weakness" on the part of the Labour leader that he has failed to rule out such a deal.
"Voters can only conclude that he would be prepared to trade Britain's national security just to get his hands on the keys to Downing Street," he will say.
"The future of our country's security will be on the ballot paper. There will be a clear choice between a Conservative government that will put our national security first. Or Ed Miliband, a man so desperate for power he is ready to barter away our nuclear deterrent in a backroom deal with the SNP."
Coaker hit back, saying: "As Ed Miliband has already made crystal clear, national security is not a matter for negotiation. We support renewal of Trident along with a renewed focus on multilateral disarmament.
"Labour is committed to maintaining a minimum, credible, independent nuclear deterrent, delivered through a continuous-at-sea deterrent. This is not up for negotiation with any party.
"The Tories are resorting to the language of smear. They are increasingly desperate as their campaign lurches on in confusion and chaos."
In his speech, Fallon will liken the SNP's approach to defence to a "student protest group", warning that it would put the security of the whole country at risk.
"The SNP's childlike world view would sacrifice the long-term security of the UK and play into the hands of our enemies. The nuclear deterrent protects all of Britain and the SNP represents a separatist threat that would dangerously weaken our collective defence," he will say.
"When Britain faces nuclear blackmail by rogue states, this self-indulgent approach is more suited to a student protest group than a party of government. Britain doesn't pay ransoms and the Conservatives would never bow to demands from Scottish separatists."
Today also sees the final deadline for nominations for General Election candidates, which closes at 4pm.