Many a commentator has suggested that heavy losses for Labour at next week's local elections could spell the end of Tony Blair's stay at 10 Downing Street. But Mr Blair's challenge is actually fairly easy.

He enters these local elections with one very simple advantage - Labour did very badly when the seats up for grabs next week were last contested. Just over a quarter of the 4,360 seats at stake, including all those in the 36 metropolitan districts outside London, were last contested in 2004 - when Labour had its worst local election performance since Harold Wilson's leadership in the late-1960s.

Indeed, if Labour had done as badly in last year's general election as it did in the 2004 local elections, the party would have been left with a quarter of the vote, trailing in third place behind the Liberal Democrats.

True, nearly all of the remaining seats at stake were last fought over in a rather better year for Labour - 2002. While the party also trailed the Conservatives four years ago, it did win the equivalent of around a third of the national vote. In these seats today it would seem Labour is more vulnerable.

However, of the nearly 1,300 seats last won in 2002 that Labour will be defending on 4 May, two-thirds are in London. Meanwhile, as it happens Labour performed a point or two worse in London in 2002 than it did outside the capital. So even here Mr Blair has the good fortune primarily to be defending a far from distinguished record.

Heavy Labour losses next week should be almost inconceivable. Even if the party were to do no more than repeat the disastrous performance of two years ago, it should still not lose much more than 150 seats. Such a figure seems unlikely to excite the headline makers, even though it would in fact be a sign that Labour is in far deeper trouble a year into this parliament than it was a year into the last one.

Of course there is a potentially nasty downside for Mr Blair. Losses much above the 250 mark will suggest that the party has turned in an even worse performance than in 2004.

Meanwhile, if Labour is to demonstrate that it is in no worse shape than it was a year into the last parliament, the party will actually need to make net gains - at least 50 or so among those seats contested in 2004.

Still, despite its weak performance in London in 2002, it is in the capital that Labour has most to lose. Here all of the seats are up for grabs in all of the 32 boroughs. In contrast outside London only a third (or occasionally a half) of each council's seats are being contested. As a result overall control of the London boroughs is far more likely to change hands than control of councils elsewhere.

Control of some half-dozen of the 15 London boroughs currently run by Labour rests on an electoral knife-edge. A repeat of the 2004 polls would see the party lose control of Bexley, Croydon, Hammersmith, Merton and perhaps even Brent. Camden and Hounslow could also be vulnerable. "Labour loses grip on the capital" is perhaps the one headline that Mr Blair really does have to fear.

In contrast outside London even doing as badly as in 2004 would only result in Labour losing a few councils. Crawley, Derby and Stoke could be vulnerable.

The corollary of Mr Blair's good fortune is of course that David Cameron has a tough task registering the 300 or so gains he needs to show that his party has turned the corner. For this the Tories must target both Labour and the Liberal Democrats.

In contrast to Labour, the Liberal Democrats are defending relatively strong performances in 2002 and - especially - 2004. So the party might yet frustrate Mr Cameron by balancing losses in some quarters with gains in others.

John Curtice is a professor of politics at Strathclyde University