A combined Labour, Liberal Democrat and SNP coalition would have the best chance to form a working government in May, a leading pollster has predicted.

The forecast came just a day after Lib Dem leader Nick Clegg said he found it difficult to imagine sitting around a cabinet table with the nationalists.

You Gov founder Peter Kellner also predicted that the Tories and Lib Dems would just have the 323 seats to have a majority, but even Tory MPs admit that could prove unworkable if even one MP falls ill.

The SNP also received a boost to their campaign to be included in the TV leaders' debate as a new poll by Tory grandee Lord Ashcroft put them on 5 per cent of the UK-wide vote.

The Greens, also battling for a TV spot, were on 11 per cent, while the Tories led Labour by one percentage point at 29 to 28 per cent.

There was a blow for Labour, however, as another respected pollster suggested the party's vote was being overestimated - and support for the Tories and the Greens underestimated.

That prediction came hot on the heels of the second poll in two days that suggested Labour was staging a recovery in electoral support in Scotland.

Rob Hayward, the polling expert and former Tory MP, also questioned whether there would be a surge in support for the SNP in May.

He pointed to mixed results for the SNP in Scottish council by-elections since September's referendum.

Whether or not there would be a huge push in SNP support in a general election remained an "unanswered question", he said.

A poll by Survation was the second in two days to show Labour appearing to close the gap on the SNP north of the border.

But the poll also showed that the option Scots would plump for most is a Labour coalition with the SNP at Westminster.

Another poll at the weekend suggested that such an option could create a large headache for Labour leader Ed Miliband, as it would have the support of just 14 per cent of voters across the UK.

At the weekend Mr Clegg said that he found it "difficult" to envisage entering a coalition with the SNP.

He declared that he would not sit around a Cabinet table with Nigel Farage's Ukip.

But he was more circumspect when it came to the SNP.

Asked whether he would enter government with the party, he said: "I find it very difficult to imagine the circumstances in which I would ever do that."

The Survation survey found that Scots second choice of government was a Labour majority, followed, in third place, by a Tory majority.

It also showed a slight narrowing in the huge poll lead the SNP has had over Labour for weeks.

A total of 46 per cent said they would vote SNP on May 7, down two points from the last such poll in December.

Support for Labour also rose by two points to 26 per cent.

SNP Westminster Leader Angus Robertson MP said: "For the SNP to be at 5 per cent in UK-wide polling - when Scotland makes up only just over 8 per cent of the UK population - is remarkable, and reflects the other excellent poll findings over the weekend."

Scottish Lib Dem leader Willie Rennie said that the SNP "are trying to win independence by the back door so would not act as a stable, progressive force in Government."

A Scottish Labour Party spokesman said: "Scottish Labour have made a fresh start and Scots are willing to listen to us again."

Lord Ashcroft's poll also asked focus groups what animal they linked with party leaders.

David Cameron was a fox or a giraffe "looking down on everybody", Nigel Farage a peacock or a weasel, Mr Clegg a 'chihuahua in Cameron's handbag" and Ed Miliband "one of those animals that, when you go to the zoo, you're not bothered whether you see it or not."