Ex-premier Tony Blair returned to Britain's campaign trail yesterday to help his embattled successor Gordon Brown, struggling to keep his party's re-election hopes alive six days before national polls.

A major gaffe and a poor performance in a key pre-election TV debate have left Mr Brown's Labour Party trailing in third place in polls, while boosting the hopes of his main opposition rival, Conservative leader David Cameron.

Mr Blair, making only his second appearance of the campaign so far in a London health centre, insisted Labour still have a chance of clinging on to power after 13 years in office.

"I believe Labour has every chance of succeeding," he said, adding: "When you get into the final days people will really focus their minds on who's got the best ideas for the future."

Asked if Mr Brown had failed as premier, Mr Blair said: "No, I don't think he's failed at all."

Less than a week before the May 6 ballot, Mr Brown battled to move on from a blunder in which he branded a pensioner as a "bigoted woman" - but his performance in the last of three TV debates failed to inspire.

Instant polls taken after the final US-style TV showdown showed Mr Brown trailing Mr Cameron by more than 10 per cent, while the Liberal Democrats' Nick Clegg came second.

A newspaper poll on voting intentions also confirmed the Conservatives are still in the lead - the YouGov survey for the Sun had the Tories on 34 per cent, Lib Dems on 28 per cent and Labour on 27 per cent.

Mr Cameron vowed yesterday there would be no complacency.

"I do not take anything for granted and we have got to fight a very hard campaign in these last six days to really win people over and say: change is possible, change can happen."

Mr Brown, unveiling a new campaign poster, acknowledged that, if opinion polls remain as they are, he could be out of power by next Friday.

"If things stay the same way, then the Conservatives and possibly the Liberals could be in a government in a coalition together," he told reporters in Birmingham, central England, where the last debate was held.

But he said: "We will continue to fight for the future of this country until the very last second of this election campaign."

The Lib Dems, long the third party in British politics, have surged spectacularly since the first TV debate on April 15, leapfrogging Labour into second place in most opinion polls.