LABOUR has claimed Nicola Sturgeon had effectively ruled out any informal coalition with an Ed Miliband administration after the General Election in the wake of an alleged Tory smear campaign around Trident.

The Conservatives accused the Labour leader of preparing to "stab the UK in the back" to secure a Labour-SNP alliance, centring on the SNP's call for the UK's nuclear deterrent on the Clyde to be scrapped.

In response, the First Minister said that the price of an informal coalition with Labour would include that the ageing system was not renewed.

Last month Mr Miliband said he would not enter formal coalition with the nationalists, but did not rule out confidence-and-supply - that would see the SNP support Labour on certain key votes, but not be in a formal Coalition. Labour yesterday, however, said that that was exactly what the SNP leader had done.

On the campaign trail Ms Sturgeon said: "Both Ed Miliband and I have said that coalition is unlikely; any confidence-and-supply arrangement would require the non-renewal of Trident."

Labour's Anne McGuire said that her party had made its position clear on the issue.

"A Labour government won't do any deals with the SNP on this issue or any other issue for that matter," she said.

Ms Sturgeon's comments showed that the SNP leader "clearly had no intention for her MPs to back Labour in power - as their get out clause is to make impossible demands about future investment on Britain's nuclear deterrent," she added.

Confidence-and-supply is a system under which a smaller party backs another during budget and other votes in return for policy concessions.

Labour also hit out at the Tories insisting that it was committed to the renewal of the system and its continuous -at-sea capability.

Earlier the Conservative Defence Secretary Michael Fallon was accused of demeaning his office by claiming Mr Miliband would stab the UK in the back.

He said: "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."

Another Tory minister Nick Boles also claimed that Russian leader Vladimir Putin was hoping for a Labour-SNP alliance after May, because of the effect he said such a grouping would have on security.

Mr Miliband denounced the comments as a "desperate smear".

The Labour leader described Mr Fallon as a decent man but questioned whether the attack was a sign of desperation from a Conservative campaign that feels it is losing the General Election.

He added that the Defence Secretary had "demeaned himself and demeaned his office".

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 Ms Sturgeon's offer to help make Mr Miliband Prime Minister as as "arrogant as it was terrifying".

The row comes as the Conservatives step up their rhetoric against any Labour- SNP co-operation 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.

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."