Labour are worried about the Liberal Democrats. They fear that Labour voters who are unhappy with Tony Blair's handling of the Iraq war will switch to the Liberal Democrats because of Charles Kennedy's opposition to the war. After all, as compared with 2001 it is the Liberal Democrats who are up in the polls, not the Conservatives.

Labour are worried about the Liberal Democrats. They fear that Labour voters who are unhappy with Tony Blair's handling of the Iraq war will switch to the Liberal Democrats because of Charles Kennedy's opposition to the war. After all, as compared with 2001 it is the Liberal Democrats who are up in the polls, not the Conservatives.

The NOP for The Independent poll earlier this week underlined Labour's fears. Amongst those who felt Mr Blair's decision to go to war was wrong, Labour's vote was down nine points on how people voted in 2001, but by only three points among those who believe he was right. Meanwhile, Liberal Democrat support was up seven points in the former group, down five in the latter.

The explosive entry of Iraq into the campaign this week has only increased Labour's concern. Hesitant Labour voters have been reminded of the controversy about the war just when they are deciding how to vote. Perhaps the reminder will encourage them to defect.

But Labour reckon that if there is one thing these voters fear more than the re-election of Mr Blair with another large majority, it is that Michael Howard should become prime minister. So every time Charles Kennedy has threatened to do damage in this campaign, Labour spokespersons have uttered one simple mantra. ''If you vote Liberal Democrat you could let Michael Howard in." Indeed yesterday Mr Blair claimed that Mr Howard would win if just one in 10 Labour voters switched to the Liberal Democrats.

Labour's claim has some truth in it. If people switch from Labour to the Liberal Democrats then the Conservatives would start to pick up Labour marginals without themselves winning a single extra vote. Moreover if enough people made the switch Labour could lose its overall majority. But the swing required, 11.5 per cent, is the equivalent of one in four Labour voters defecting, not one in 10. So, the Liberal Democrats could potentially threaten Labour's security of tenure, but the swing required is well above the figure suggested by the Prime Minister.

However, it does not follow that if Labour has lost its majority that the Conservatives have secured one. If there were a 11.5 per cent swing from Labour to the Liberal Democrats the Conservatives would still only have just over 200 seats, far short of the 324 needed for a majority. Britain would simply have a hung parliament.

Indeed, there is no uniform swing from Labour to the Liberal Democrats that produces a majority Conservative government. If the Conservatives remain stuck on 33 per cent of the vote about the best it could ever possibly achieve is around 230 seats. And that eventuality would happen only on a swing of 15 per cent or more from Labour to the Liberal Democrats, the equivalent of one in three Labour voters defecting. In fact there is virtually no swing from Labour to the Liberal Democrats that even enables the Conservatives to become the largest party let alone gives it a majority. It is also impossible that any such swing would enable the Conservatives to secure an overall majority even if they were up three points or so on 2001.

The reason is simple, So many Conservative MPs face a second-placed Liberal Democrat challenger that swings of much above 15 per cent from Labour to the Liberal Democrats would begin to result in the mass decapitation of Conservative MPs.

The real truth about the likely effect of switching from Labour to the Liberal Democrats is rather different from the picture being painted by Labour - it could eliminate Mr Blair's majority but it alone cannot let Mr Howard in.

John Curtice is one of Britain's most respected political commentators and has been writing about the country's electoral system and voting behaviour for 25 years. He is professor of politics at Strathclyde University and co-author of "The Rise of New Labour".