Prime Minister Tony Blair and Chancellor Gordon Brown have stepped up their efforts to persuade Labour members not to choose Ken Livingstone's as the party's candidate for London mayor.

But Mr Blair was jeered and hissed as he told a meeting of the party faithful that it was time to "get serious" and reject Mr Livingstone in favour of Downing Street favourite Frank Dobson.

"This is a serious election, it matters," Mr Blair told the audience of more than 1,000 in central London.

"We cannot afford a return to gesture politics."

In a direct reference to Mr Livingstone, he added: "We cannot afford the situation where someone is calling for the sacking of the chancellor or where someone is calling for people who are rioting on the streets of Seattle to be people we support."

Responding to heckles, he added: "The fact is that Ken did call for Gordon to be sacked and that is not sensible is it?"

The prime minister went on: "We've spent 18 years in opposition and I never want to go back."

Mr Brown joined in the attack saying Mr Livingstone's policies would deter investments and damage job prospects.

"For 20 years we marched for jobs, we demonstrated for jobs, we campaigned for jobs and we are now in a position to create jobs," he said.

He added: "We won't do it by going back the old divisive, sterile self-defeating policies of the 1980s."

However, some members of the audience shouted that it was time to give Mr Livingstone a chance.

One woman told the prime minister to stop "slagging off" Mr Livingstone on behalf of Mr Dobson.

And another warned him that attacking Mr Livingstone was the best way to get the former leader of the Greater London Council elected.

Mr Livingstone, who was not at the meeting, called on the prime minister and the chancellor to stay out of the election.

"Londoners don't need to be told who to vote for," he said.

But speaking on Newsnight, he claimed that privately Downing Street had accepted that he might win the nomination and was looking at ways in which it could work with him.

"I am looking forward to working with Tony Blair," he said.

Earlier, in an article in a newspaper article, Mr Brown warned that the left-wing MP's plans for financing London Underground could mean a £10bn debt burden for the capital.

The chancellor also said that Mr Livingstone's policies on taxation could mean businesses refusing to locate in the capital.

Mr Brown said in the article: "For Ken Livingstone the Tube has become an issue of economic dogma, ignoring the realities of getting the investment needed to improve everyday life for Londoners."

He said Mr Livingstone's plans for public sector bonds to finance Tube redevelopment would saddle London with up to £10bn of debt.

Mr Brown also heaped praise on Mr Dobson.

"He was a central member of the team which beat the Tories," the chancellor wrote.

"And I've seen the way he fights his corner and argues his case. London could ask for no more effective champion."

Labour members in London will have to choose between Mr Livingstone, Mr Dobson or former minister Glenda Jackson.

The party is sending out voting papers to its 60,000 members in the capital next week.