Despite all Labour's travails, there have always been doubts about the strength of the Conservatives' electoral prospects. It has never been clear that the party had "sealed the deal" with the electorate.

For a start, the party has rarely secured much more than 40 per cent in recent months. That was hardly the performance of a party that had the enthusiastic support of a large section of the electorate.

Meanwhile, when, as they do each month, ComRes have asked people whether they think of themselves as a Conservative, Labour or Liberal Democrat, those declaring themselves Conservative have typically been no more numerous than Labour. This suggested the Tory lead was built on soft, potentially temporary support rather than loyal support that would necessarily be sustained all the way through to next May.

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Now those doubts have been underlined. There is still no sign of a Labour recovery; Gordon Brown's Government remains deeply unpopular. Nevertheless, Tory support has fallen by three points. As a result, at 10 points, the party's lead is now no longer sufficient for Mr Cameron to be sure he would secure an overall majority.

Individual polls can, of course, sometimes be misleading. But this is no isolated, unusual result. It confirms a trend of slipping Conservative support that was apparent in many polls throughout last month. Mr Cameron really does have reason to worry. He does, after all, face plenty of competition in the scramble for the votes of the disaffected and disappointed. The classic party of protest, the Liberal Democrats, have been polling more strongly, at around 20 per cent, for some months now. Meanwhile smaller parties continue to register unprecedented levels of support.

Mr Cameron must wonder in particular how many of the 6 per cent of voters who say they would vote for Ukip might otherwise have been Conservative supporters. He must be hoping the new Ukip leader, Lord Pearson, will prove a less effective than his predecessor, Nigel Farage.

Still, Labour would be unwise to derive too much comfort from this poll. It still suggests Mr Cameron will lead the next government, albeit perhaps from a more precarious position than he would like. And if Lord Ashcroft's efforts in marginal seats pay off, today's 10-point lead might still be enough for a Tory majority. Even so, evidently the outcome next May is far from being a foregone conclusion.

John Curtice is Professor of Politics at Strathclyde University