ED Miliband has ruled out any kind of formal deal with the SNP in a move that would leave a minority Labour government facing a daily balancing act to implement key parts of its programme.

The Labour leader had previously rejected the possibility of a coalition with the Nationalists but went further yesterday, repeating three times during a TV interview that he would not enter a looser "confidence and supply" pact with Nicola Sturgeon's MPs.

His comments came as senior Conservatives and Liberal Democrats raised questions about the "legitimacy" of a minority Labour government supported by the SNP.

Theresa May, the Home Secretary, warned people in England would find it "very difficult to accept" a governing alliance between Labour and the SNP.

Nicola Sturgeon, the SNP leader, said Mr Miliband would be forced to "change his tune".

She warned he would have to work with other parties or face claims he preferred David Cameron to "waltz back into Downing Street".

ElectionForecast, which provides the BBC's daily "Newsnight Index" prediction, said the Conservatives were on course to emerge as the biggest party with 283 seats, ahead of Labour on 271.

The SNP's projected 47 MPs would stop Labour being the biggest party but could not provide enough support to prop up Mr Miliband on their own, with the two parties' combined total of 312 seats falling short of the 323 required for an effective majority.

The Lib Dems are forecast to win 25 seats and have indicated they would talk first to the biggest party about a possible pact. Leader Nick Clegg has, however, ruled out any multi-party alliance involving the SNP.

Ulster's DUP, to whom the Tories would look for support, are forecast to win eight seats.

Mr Miliband ruled out a "confidence and supply" deal with the SNP - in which the Nationalists would enter a formal agreement to keep Labour in government in return for specific concessions - during an appearance on the BBC's Andrew Marr programme.

Asked repeatedly about such an arrangement, he said: "I want to be clear about this: no deals, no tie-ins.

"I'm not doing deals with the SNP, I want a majority Labour government.

"If there is a labour government, it will be a Labour Queen's Speech, it will be a Labour Budget, it is not going to be written by the SNP. I can't be clearer than that."

He insisted his party could still win in Scotland, despite a new Panelbase poll which showed the SNP extending its lead over Labour to 21 points.

Mr Miliband's public rejection of any pact with the Nationalists will hearten his Scottish candidates who, as the The Herald revealed, were reassured privately about the strategy some weeks ago.

However it leaves open the prospect of five years of horse-trading with the other parties to win support for policies on a vote-by-vote basis.

Ms Sturgeon has vowed to be a "constructive and responsible" ally but could threaten to withhold support from specific Labour policies in return for concessions on others.

The Conservatives yesterday intensified their warning about a possible SNP-backed minority Labour government, claiming it would not be considered "legitimate".

Ms May said it could provoke "the biggest constitutional crisis since the abdication," when King Edward VIII gave up the throne in 1936.

She added: "There would be a very real feeling this was something people did not want to see, had not voted for and would find very difficult to accept.

"It would raise difficult questions about legitimacy. A lot of English people would question that."

The Tories unveiled a poster depicting Alex Salmond as a pickpocket, while London Mayor Boris Johnson claimed Mr Miliband would have the SNP "crouching on his back like a monkey".

Over the weekend, Mr Clegg also warned a "coalition of the losers" after May 7 could lack "legitimacy".

He said: "You cannot provide stability, you can't take difficult decisions, if people are constantly questioning the birthright of a government."

Ms Sturgeon said: "It's the people who are in charge and the politicians have to respect the democratic wishes of the people.

"If Ed Miliband doesn't get a majority, as the polls are all saying he won't, then he'll have to work with other parties.

"I suspect Ed Miliband will change his tune once the votes are cast."

She warned Mr Miliband against letting David Cameron "waltz back into Downing Street," adding: "I don't want that and the SNP will use our votes to stop a Tory government getting off the ground if there is that anti-Tory majority."

Ms Sturgeon will pick up the message again today when she campaigns in Kilmarnock.

Speaking ahead of the visit, she said: "It is clear that the Westminster parties have hit the panic button.

"Voters do not appear to trust either party with a majority.

"The SNP is being open and honest about our position, that we will work to keep the Tories out and to keep Labour honest.

"Labour would never be forgiven if they let the Tories back in in preference to working with the SNP."