Labour's attempts to warn its wavering supporters that a vote for the Liberal Democrats could allow Michael Howard into No. 10 "by the back door" was undermined yesterday in a detailed study carried out for The Independent. The study found that a swing of 11.5 per cent from Labour voters to the Liberal Democrats could deprive Mr Blair of his overall Commons majority but it would be virtually impossible for such defections - at even twice that rate - to let in the Conservatives to form a government.
John Curtice, the respected psephologist and professor of politics at Strathclyde University, who carried out the analysis, said: "Labour's claim that switching from Labour to the Liberal Democrats could enable Mr Howard to win the election is highly misleading."
Professor Curtice said: "It is even highly unlikely any swing could result in the Conservatives becoming the largest party. The most likely consequence of any large switch from Labour to the Liberal Democrats is simply nobody would have an overall majority." His study came as Labour sought to deflect attention from the row over the war in Iraq, with cabinet ministers raising the spectre of a Tory victory - made possible by disenchanted Labour voters deserting for Charles Kennedy's party.
Mr Blair led the co-ordinated campaign and it became a recurrent theme on many of the Labour campaign spots. However, the "Lab-to-Lib Dem swingometer" Professor Curtice devised shows that, even if there is a 9 per cent swing from Labour to the Liberal Democrats, Labour would still have an overall majority of 48.
Even with a massive 15 per cent swing, Labour would still be the largest party in a hung parliament with 50 more seats than the Tories. Those figures are based on the Tories doing no better than the 33 per cent share of the vote they won in 2001, which is also in line with their current opinion poll rating.
Even if the Tories improve their showing and win 36 per cent of the votes next Thursday - a three-percentage point rise at Labour's expense - and there were a 9 per cent swing from Labour to the Liberal Democrats over and above that, Labour would be the largest party in a hung parliament with 43 seats more than the Tories. The "Lab- to-Lib Dem" swing would have to be a huge 15 per cent before the Tories became the largest party - but they would be outnumbered by the combined forces of the other two parties.
The study found that the Tories come nowhere near to passing the winning post of 324 seats they would need to form a government. Crucially, if enough people switched from Labour to the Liberal Democrats, Mr Kennedy's party would start to win seats instead of the Tories -so the result would be a hung parliament rather than a Tory government.
In those circumstances, according to senior Labour and Liberal Democrat figures, the most likely outcome would be a deal between the two parties to keep the Tories out.
In an attempt to limit the damage after Iraq moved to the top of the election agenda, Labour is raising the spectre of Mr Howard being installed in Downing Street by next Friday. It plans a poster depicting the Tory leader and a warning that spurning Labour could allow him into No 10 "through the back door".
Mr Blair led the charge yesterday when he claimed: "It only takes one in 10 of our voters to drift off to the Liberal Democrats and you end up with a Tory government." But Professor Curtice said: "Labour would need to lose around one in four of its voters before its majority would be threatened, not just one in 10."
He also rejected another claim by Labour - that if only one in 50 Labour voters in 80 marginal seats back the Liberal Democrats or abstain, there would be a Conservative government.
Professor Curtice replied: "If Labour lose 80 seats, then Tony Blair does indeed lose his majority with just 323 seats. But 80 extra Conservative seats leaves Michael Howard with just 245 seats, still 79 short of a majority.
"It is inconceivable that Michael Howard would be able to form a government in such circumstances. Labour's message seems to confuse the possibility of their own 'defeat' with a 'victory' for Mr Howard."
Mr Kennedy said: "There is no chance whatsoever of the Conservatives getting in by the back, the side or any other door because they have lost this election and people know they have lost.
"After eight years in power with three-figure majorities and a benign economic backdrop, if the best you can turn round to the country and say is, 'Well, you must not vote for the Liberal Democrats for fear of something worse,' that just shows you what thin ice Tony Blair is on."
Labour stood by its claims last night. A party source said: "If people value progress and want to accelerate change, they can't afford to abstain or flirt with the Liberal Democrats. If you value it, vote for it. That is the clear message we will be taking to every corner of the country in the days to come."
Although the Liberal Democrats do not campaign openly for a hung parliament, they suspect some voters would prefer that outcome to a big Labour majority. A poll by ICM showed that, if people thought Mr Kennedy's party could win in their constituency, 39 per cent would vote for it, with 31 per cent backing Labour and 26 per cent backing the Tories.