Conservative Defence Secretary Michael Fallon has been accused of demeaning his office after he claimed Ed Miliband would "stab the UK in the back" to do a deal with the SNP over Trident.
The Tory minister claimed Mr Miliband would do anything to get into Downing Street as he accused him of betraying his own brother to become Labour leader.
Mr Miliband denounced the comments as a "desperate smear".
In a speech in central London Mr Fallon also accused the SNP of a "self-indulgent... student politics" approach to nuclear weapons that risked sacrificing national security.
He described First Minister Nicola Sturgeon's offer to help make Ed Miliband Prime Minister as as "arrogant as it was terrifying".
The row comes as the Conservatives step up their rhetoric against any Labour- SNP alliance in the event of a hung parliament in May.
The party hopes the attack will help to shore up Tory votes south of the border
But Mr Fallon faced accusations that he had gone too far in his deeply personal remarks about the Labour leader.
Earlier, he had warned of Mr Miliband: "We saw in that Labour leadership election just what he would do to get into power. You cannot be sure what kind of backstairs deal he would do after the election."
"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."
Mr Miliband described Mr Fallon as a decent man but hit out at the Tory minister's methods and questioned whether the move was a sign of desperation from a Conservative campaign that feels it is losing the General Election.
In his speech Mr Fallon said that it was clear that Mr Miliband "will do anything to get into Downing Street".
He added that the Defence Secretary had "demeaned himself and demeaned his office".
"Imagine Ed Miliband limping into office aided by the crutch of the SNP," he added.
The nationalists aim to rid Scotland of nuclear weapons was a "self-indulgent approach more suited to politics of student politics," he said.
This "naive" politics would "sacrifice the security of the UK and play into the hands of our enemies," he added.
He claimed that replacing the ageing nuclear deterrent on the Clyde would lead to "negligible" cost savings, less than will be spent on the new high-speed rail scheme HS2.
Mr Fallon also denied that his party had given into Liberal Democrat demands over Trident during coalition negotiations in 2010.
The Coalition Agreement delayed the 'maingate' Commons vote on the weapons system until 2016.
But it did allow preparatory work on a replacement for the nuclear deterrent to be undertaken by the Ministry of Defence (MoD).
Mr Fallon also appeared to rule out any deal with the Lib Dems if the party sticks to its aim to cut the number of Trident nuclear submarines below four.
The Defence Secretary said that the Tories were "not prepared to do a deal with the Lib Dems or any other party" on the issue of four boats.
Labour insists that they are also committed, like the Tories, to keeping a continuous-at-sea nuclear deterrent.
The party has not pledged that that would include four boats, but there are signs it is minded that way.
During his speech Mr Fallon also described Faslane, Trident's home base, as the UK's "peace camp".
He warned: "We cannot know now what nuclear threats may emerge in 2030s and 2040s".
Scottish National Party Defence Spokesperson and Campaign Director Angus Robertson said: "Trident is utterly irrelevant to the defence and security challenges we face in the 21st century, and therefore the appropriate number of Trident submarines isn't four or three - it is zero."
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