A Populus poll for The Times, taken over the weekend, suggests that Labour has strengthened its support since the election was called a fortnight ago, while the Tories have slipped.

Labour is now on 40 per cent, up 3 points compared with two weeks ago, and in line with recent non-internet polls. The Tories are 4 points down over the same period at 31 per cent, with the Liberal Democrats 2 points up at 21 per cent.

If these figures were repeated on May 5, Labour would have a Commons majority of more than 100, compared with 165 at the 2001 election. These ratings take into account the fact that Conservative supporters say that they are more certain to vote than Labour’s.

The poll has Mr Howard trailing behind both Tony Blair and Charles Kennedy in his ability to impress voters. Mr Kennedy is ahead among floating voters who may change their preference before the election.

In a further blow to Mr Howard, the poll finds the public highly sceptical about promises to improve public services or to reduce crime. Despite Tory pledges to cut taxes by £4 billion, half of voters believe that they would be increased if Mr Howard’s party won.

An NOP poll for The Independent put Labour on 37 per cent, 1 point down on the previous week, with the Conservatives and Lib Dems unchanged on 32 per cent and 21.

The findings came as a number of senior Conservatives levelled private criticism at the Howard campaign, one calling it too much of a “one-man band” and another saying that it focused excessively on shoring up the Tory core vote.

One senior Tory said: “This is a core vote campaign, along the lines of 2001. It is much more professionally organised, thanks to Lynton Crosby (the Australian strategist running the campaign). But in Australia Mr Crosby does not have to worry about the threat from a third party. What seems to be happening here is that when we make a hit on Labour, the Liberal Democrats get two thirds of the benefit.”

There is no sign of a wobble inside Mr Howard’s tight campaign team. Members of the close-knit group around him said last night that there would be no change of strategy.

There is disbelief that the Labour vote is holding up as well as the polls suggest. Senior Tories claim that polling errors made in the past three elections, which overestimated the extent of Labour’s lead have not been rectified.

Liam Fox, the party co-chairman, will today present to colleagues his view of what the polls are saying. He will say that there are “mixed messages” from the various polling companies, and that his voting analysis, along with feedback from activists, suggests “a tight election race all the way until polling day”.

Other advisers say that campaign messages on immigration and crime have been playing well on the doorstep, and that the focus on the ground would shift to tax cuts this week after Sunday’s announcement of tax breaks on private pension contributions.

But another senior Conservative told The Times that he felt the campaign was “too harsh” and said too little on issues such as schools and hospitals.

One frontbencher said that Mr Howard was dominating the campaign. Oliver Letwin, the Shadow Chancellor, was used more than most but “the rest of us are pretty invisible”.

“If Michael is not popular, should we not be showing ourselves off as more of a team?” he said.

A Times survey of the number of times that members of the Shadow Cabinet have been mentioned in the national media since the election was called bears out criticism that the campaign is centred on Mr Howard alone.

While the party leader has had 1,130 mentions, his deputy, Michael Ancram, has had 11, Andrew Lansley, the health spokesman, 37, and Mr Letwin 189, compared with 1,018 for Gordon Brown. Caroline Spelman, the party’s local government spokesman and one of its most effective communicators, has had 19 mentions. But this is more than Theresa May, one of the party’s best-known performers, who has 17. David Davis, the Shadow Home Secretary, has had 75 mentions.

Some Conservatives said that the unexpectedly prominent role taken by Mr Brown had caused problems. “Some of Labour’s most disillusioned core voters who probably were not going to vote because they feel so let down by Blair may have been cheered up by Gordon Brown taking centre stage all of last week,” one said.

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