Magnus Gardham

Michael Settle

THE SNP is in an "unholy alliance" with the Conservatives to prevent Labour coming to power at the General Election, Ed Miliband has claimed, as a new poll suggested his party was heading for disaster in Scotland on May 7.

But the Nationalist leadership branded the assertion that a vote for the SNP would put David Cameron back in Downing Street as "the big lie in British politics"; claiming Labour was being dishonest with voters as a vote for progressive politics in Scotland could lead to a non-Tory government in London.

At a campaign event in Clydebank, the Labour leader accused the Nationalists of boosting the Conservatives' campaign after Alex Salmond said the SNP would "hold the power" and dictate the Budget if a minority Labour government required its votes to survive.

The former First Minister's comments, at the weekend, were immediately seized on by the Tories who produced an election video depicting Mr Salmond playing a pipe and Mr Miliband dancing.

The Tories' claim that the SNP would "call the tune" if Labour fell short of an overall majority have been credited with shoring up support for David Cameron south of the Border.

Mr Miliband used the campaign event to claim the Nationalists and Tories were in league with each other.

Asked if Mr Salmond "had him over a barrel," he told an audience of supporters: "There seems to now be an unholy alliance between the Conservatives and the SNP to carry on a Tory Government.

"Frankly, Alex Salmond is at it again, and it's a combination of bluster and bluff.

"I gather he has got a book to sell.

"I'll tell you who is going to be writing the Labour budget, it's me and Ed Balls.

"It's not going to be Alex Salmond, not in a million years."

The Labour leader went further than previously in talking down the possibility of SNP MPs holding sway in a hung parliament.

Hinting that a minority Labour government would defy the SNP to vote against it, he said: "It will be for other parties to decide how to vote on a Labour Queen's speech.

"I couldn't be clearer, I'm putting forward a Labour manifesto I want to implement."

Labour, he insisted, could win a majority.

But Angus Robertson, the Nationalist leader at Westminster, hit back at the assertion that if Labour did badly in Scotland and the SNP did well, it would boost the Tory leader's chances of staying in power.

"This is the big lie in British politics," declared the Moray MP.

"Any student of the UK political system understands that governments are formed by parties that command a majority in the House of Commons; that is the beginning and the end of it. All of the rest of it - these are claims that are even printed on Labour election leaflets - are lies.

"The constitutional position is absolutely clear; if there is a majority in the Commons for a non-Conservative government, that is the government that will be formed. The Labour Party needs to be honest about this because people in Scotland are not foolish, they understand this, and to be telling them anything different is dishonest and it's counter-productive to the Labour Party because it's insulting people's intelligence."

Asked, given the SNP would not do a deal with the Conservatives but it might with Labour, that the Nationalist leadership would therefore prefer to see Mr Miliband in Downing Street than Mr Cameron, he replied: "We definitely don't want to see David Cameron back in Downing St and the best option after that is for a UK Government dependent on the positive influence that the SNP can exercise."

Accepting that Scottish independence was not on the ballot paper, he added: "I share the views of the majority of people in Scotland when asked the question, which is that there should be co-operation between the SNP and the Labour Party."

Meantime, a new ICM poll has put support for the SNP on 43 per cent and Labour on 27 per cent, virtually unchanged since December.

On a uniform swing, the SNP would win 43 seats to Labour's 12; making it almost impossible for Mr Miliband to become Prime Minister without the support of another party or parties.

But an analysis by polling expert Professor John Curtice suggested Labour was in an even worse position with just two seats in Scotland; the same number as the Tories and Lib Dems. The SNP would take the remaining 53.