Miliband’s comments come as senior party source claims Tories and SNP want to damage Labour regardless of wider consequences

Ed Miliband went on the offensive against the Tories and the SNP as he warned that the two supposed enemies have formed an “unholy alliance” to ensure that David Cameron remains as prime minister after the general election.

ICM poll: Labour faces wipeout in Scotland after new leader fails to dent SNP support Read more

As a Guardian/ICM poll showed that Labour is on course to surrender Scotland to the SNP – with a rump of 12 MPs north of the border – Miliband hit back at what the Labour leadership regards as a marriage of convenience to promote Tory and SNP interests by crushing his party.

“There seems to now be an unholy alliance between the Conservative party and the SNP to carry on a Tory government and frankly Alex Salmond is at it again,” Miliband said after the former SNP first minister said he would seek to rewrite a Labour budget.

Speaking in the Clydeback and Milngavie constituency, a safe Labour seat now under threat from the SNP, Miliband added: “It’s a combination of bluster and bluff; I gather he has got a book to sell.”

Miliband decided to intensify his attack on the SNP and the Tories amid deep fears that Labour is facing a grave threat from the joint, though apparently uncoordinated, tactics of the two parties.

The SNP points out that it expects to be able to dictate terms to a Labour-led government in a hung parliament.

The Tories respond to such interventions by saying that Miliband would be beholden to the SNP, as they did on Sunday when the Conservatives released a video showing the Labour leader “dancing to Salmond’s tune”.

The tactics are dangerous to Labour in Scotland and the rest of Great Britain. The SNP’s claim that it would influence a Labour-led government in an informal arrangement in a hung parliament is designed to undermine the traditional Labour claim that a vote for the SNP would lead to a Tory government.

The Tory warnings about the dominant role the SNP could play is designed to illustrate the Conservative claim that voters face a choice between competence and chaos at the election.

Jim Murphy, the leader of the Labour party in Scotland, highlighted Labour nerves when he suggested that time was closing in on the party.

Murphy told the Guardian: “You can raise more money, you can make new policies but you can’t make more time. So we’re in a sprint to cover a lot of ground but, as you know, I’m a marathon runner. I just have to sprint for a full marathon.

“We don’t dispute we’ve got a massive challenge. I read the opinion polls, I meet enough voters, but Scotland is a lot less angry than it was in September, October or November. People have gone from shouting at the Labour party and are starting to listen to us. Now that they’re listening, can we convince them?”

Murphy spoke out as a Guardian/ICM poll confirmed the SNP’s dominant position despite losing the independence referendum in September. Nicola Sturgeon’s SNP is way out in front on 43%, exactly the same level of support it enjoyed in ICM’s last Scottish poll for the Guardian in December.

Scottish Labour languishes 16 points behind on 27%, an advance of only one point since Christmas and the arrival of their new leader. The Liberal Democrats are on an unchanged 6%, while the Scottish Conservatives inch up one to 14%.

Pressed on the findings of the latest poll, Miliband said: “You don’t blow the whistle on a match before the game is over. I’m not going to do that because there are six-and-a-half weeks to go and six-and-a-half weeks for people to make up their minds.”

The Labour leader sought to shut down the Tory attacks last week by ruling out a coalition with the SNP. But he declined to rule out a less formal arrangement, prompting renewed Tory warnings of a post-election deal between Labour and the SNP.

A senior Labour source said that the Tories and the SNP were seeking to damage Labour, regardless of the wider consequences. The source said: “The SNP and the Tories are dancing to the same tune.

“The SNP say: ‘We will make sure Labour is leftwing’ and the Tories then run adverts about it. The message discipline between the Tories and the SNP is quote extraordinary. They appear not in cahoots.

“But the SNP and the Conservative party have a shared interest in deeply damaging the Labour party in Scotland. The SNP don’t care whether that means the election of another Conservative government and the Conservative party doesn’t care about what that means about the UK. That shows a deep cynicism in both parties.”