At the same time the poster campaign alleging that Mr Blair would lie to win the election has been replaced after three days. The latest Times/ITV News tracker poll shows that, by a two-to-one margin, the “liar” attacks have damaged the Tories more than Labour.

Mr Howard signalled the shift in tactics yesterday morning when he said: “The British people want to move on from the debate of the past few days. Character is an important issue for them, but now they want to hear about something better.”

The posters — saying “If he’s prepared to lie to take us to war, he’s prepared to lie to win an election” — were unveiled on Tuesday. They never appeared on billboards and just featured on 12 advertising vans for three days.

The four-day tracker poll, the last quarter of which was completed on Thursday evening, shows that Labour support is remaining firm, at 40 per cent for the fourth day running, depsite this week’s attacks on Mr Blair and the renewed controversy over Iraq. By contrast, the Tories look stuck, losing a point to 31 per cent. The Lib Dems have gained a point to 22 per cent, their highest level of the campaign. Other parties are on 7 per cent.

Labour strategists are worried that further gains by the Lib Dems, at Labour’s expense, might allow the Tories to win in many of their 50 most marginal seats. But, at present, Labour support is holding up well.

These voting figures are based on those saying they are likely to vote, an implied turnout of 60 per cent, up one point from the 2001 election.

However, if only those saying they are certain to vote are included, an implied turnout of just 42 per cent, Labour would still be in the lead on 39 per cent, ahead of the Tories on 34 per cent, and the Lib Dems on 20 per cent.

This would indicate a reduced, but still sizeable, Labour majority. Populus asked about the Tory “liar” attacks. Just 21 per cent said the charge made them less likely to vote Labour, but 44 per cent said it made them less likely to vote Conservative. This includes 17 per cent of Tory voters.

By a more than two to one margin, 66 to 29 per cent, voters do not think that Mr Blair is more of a liar than other politicians. Three fifths of the public think that, by calling Mr Blair a liar, the Tories are “just resorting to name-calling and showing that they have nothing positive to say to try to win people’s votes”. When the “liar” advertisement was unveiled, it was presented as the “latest campaign poster”.

However, a spokeswoman for M&C Saatchi told The Times that the campaign was never intended to go on display on billboards.

There was evidence last night of Conservative discontent over Mr Howard’s campaign. Stephen Yorke, who was John Major’s deputy political secretary, said he had resigned from the Tory party in disgust.

Mr Yorke said: “I’ve been a member for 20 years but when I watched Michael Howard trying to exploit the war for political advantage this week, I decided that I’d had enough.”

He now plans to vote Labour in the marginal seat of Battersea. “I think Tony Blair has shown enormous political courage over Iraq and we have a lot to thank him for. I also believe the Government was right on student fees.”

For full poll details, see www.populuslimited.com

Times Recommends

• Anne Darwin's performance of a lifetime
• Renaissance for canals as new boaters come aboard
• Case study: forced to marry a cousin at 17