The Populus poll for The Times, taken over the weekend, suggests that the main shift has been from the Tories to the UK Independence Party.

Support for the Tories has been level since the summer at 36 per cent, but has fallen by two points over the past month. Labour is unchanged on 33 per cent, with the Liberal Democrats down a point at 19 per cent.

But support for other parties has risen by three points to 14 per cent, the highest in any Populus poll since the last election. The main beneficiaries of the gain have been the Greens, on 4 per cent, and the UKIP on 2 per cent. While the numbers of voters here are very small, the drift away from the main parties is potentially significant.

The poll follows defections to UKIP by some prominent Tory activitists and rumblings of discontent on the Right about Mr Cameron’s leadership over his views on green policies, tax and Europe. A recent survey by the ConservativeHome website found that, next to their own party, 43 per cent of Tory members felt closest to UKIP.

Nevertheless, the poll shows that the Tories under Mr Cameron would still enjoy a big lead over Labour led by Gordon Brown at the next general election, of 39 to 32 per cent. This compares with a 38 to 34 per cent gap last month.

The poll, the first to be undertaken after the Pre-Budget Report, coincides with a period of frantic activity by Mr Blair.

The Prime Minister, who made three keynote speeches last week as well as dashing to Washington to see President Bush, will hold his monthly press conference today.

Tomorrow he faces the last Prime Minister’s Questions of the year before travelling on Thursday to the European Council summit in Brussels. He is also expected to make an extensive trip to the Middle East before Christmas.

Mr Blair’s energetic behaviour has provoked speculation that he is preparing to call it a day earlier than expected, and that he could announce his departure early next year. However, senior party figures want him to stay on until after the Scottish and Welsh elections in May, giving his successor, almost certainly Mr Brown, a chance to start afresh.

Mr Brown’s advisers are aware of the need for him to break through public scepticism about his prime ministerial potential.

A series of heavyweight speeches on subjects beyond his Treasury brief have been designed to underline his credentials for the top job.

But advisers admit that Mr Brown’s lengthy occupation of the Treasury has reinforced a public impression of dourness which makes people feel less comfortable with him than perhaps they were with Mr Blair.

Populus interviewed a random sample of 1,513 adults aged over 18 by telephone between Dec 8 and 10. Interviews were conducted across the country and the results have been weighted to be representative of all adults. Populus is a member of the British Polling Council and abides by its rules. For more details go to www.populuslimited.com.

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