ED Miliband and Ed Balls arrive in Scotland today to capitalise on Labour's growing poll momentum by intensifying their economic attack on the SNP and underlining a post-election deal of any kind with Nicola Sturgeon's Nationalists has now been ruled out.

Three new surveys have boosted the Labour leader's hopes of entering Downing Street, placing the party between four and six points ahead of the Tories.

Another, YouGov's Nowcast survey, which questioned 19,000 people since Saturday, predicted Labour would win 277 seats compared to the Tories' 264 on May 7. It suggested the SNP would return 55 MPs.

It came as the fall-out to a bitter row over Trident renewal made left any formal arrangement between Labour and the SNP following the May 7 General Election - a so-called confidence and supply deal - off the table.

The First Minister insisted: "The SNP has made very clear that Trident is a fundamental issue, so we would never be in any deal with a Labour government, which is going to renew Trident and we would never vote for the renewal of Trident or for anything that facilitated that renewal."

In a statement, officially issued by Scottish Labour, ex-MP Dame Anne McGuire made clear that, because her party was fully committed to Trident renewal, then Ms Sturgeon had ruled out backing any future Labour government.

Dame Anne declared: "A Labour government won't do any deals with the SNP on this issue or any other issue for that matter."

This leaves the only remaining option in a hung parliament, where the Tories cannot form a government and the SNP is the third largest party, of the Nationalists agreeing to support a minority Labour administration on a vote-by-vote, issue-by-issue basis.

Today in Edinburgh, the UK Labour leader and Shadow Chancellor will join Jim Murphy to launch a combined attack on the SNP leader's commitment, made during the second televised Scottish leaders' debate, to immediate full fiscal autonomy(FFA), which the Scottish Labour leader branded a "stupid strategic error", that had left his party with "a spring in our step".

The Labour trio will insist FFA would mean an end to the Barnett Formula, that has served Scotland well, and a £7.6bn black hole in Scotland's finances.

Mr Miliband is due to say: "A £7.6bn gap; that would need to be filled with more taxes on working people or more borrowing. You can't build social justice with a £7.6bn funding gap because the burdens of it would fall on working families across Scotland."

Stressing how the Nationalists cannot say how they would fill this gap, the Labour leader will warn that it would end the decades of pooling and sharing of resources across the UK.

"It means the benefits of Labour policies, like the mansion tax for the NHS and the bank bonus tax to pay for jobs for our young people, won't be felt in Scotland.

"This strikes to the very heart of what I believe in. I will never sell Scotland short by signing up to the SNP's plans," he will declare.

A Survation poll, which puts Labour four points ahead of the Tories, was conducted following Mr Miliband's announcement that a Labour Government would abolish the 200-year-old "non-doms" tax break rule which allows people to be 'non-domiciled' in the UK because their permanent home was abroad.

A TNS snapshot poll also put the party four points ahead of David Cameron's party, while a Panelbase placed them six ahead.

Senior party figures expressed hope that Mr Murphy's robust TV performances this week would encourage Mr Miliband to take a tougher line of attack against the First Minister when the two appear alongside Ukip's Nigel Farage and the Greens' Natalie Bennett in the so-called "challengers' debate" on the BBC next Thursday night.

"I hope Ed studies the Scottish debates," said one insider. "They showed that Nicola struggles when she comes under pressure."

Another senior figure added: "The debates showed that, despite the reaction in England, in Scotland not all of the public is worshipping at the SNP's feet."

The past 24 hours of campaigning have been marked by bitter exchanges on Trident renewal after Tory high command launched a direct personal attack against Mr Miliband.

Michael Fallon, the Defence Secretary, accused the Labour leader of planning to "stab the United Kingdom in the back" over the renewal of the Trident, comparing the prospect to Mr Miliband's victory over his brother David in the Labour leadership election.

The Labour leader, who reiterated his party's commitment to renewing Britain's nuclear deterrent, accused the Secretary of State of having "demeaned himself and demeaned his office" by his personal attack

He claimed it showed the Conservative campaign had "descended into the gutter".

The strength of the Tory attack is thought to be linked to Conservative supporters south of the border raising strong fears about the prospect of a Lab-SNP alliance.

A senior Tory source said: "Scotland is playing big on the doorsteps in England." One frontbencher noted how the issue of a Miliband-Sturgeon pact was "a number one issue for voters in London".