The leading candidates for the powerful post of London mayor clashed at a Reuters Newsmaker event on Tuesday over their competence to run the capital's 11 billion pound budget.

Ken Livingstone, a one-time socialist firebrand seeking a third four-year term as mayor, belittled his Conservative rival Boris Johnson - a former journalist - for a lack of administrative experience.

The winner of the May 1 election will be responsible for preparations for the 2012 London Olympics as well as overseeing policing, housing and the city's sprawling transport network.

Livingstone, sniping at his Eton-educated rival, said Johnson's hardest decision while editing the right-leaning Spectator magazine had been where to have lunch.

"You know what you are going to get with me," said Livingstone. "We will carry on with the programmes we have set out."

"There is clearly a bigger risk with somebody who hasn't been tried and tested."

Johnson, 43, a backbench MP who says he is the only candidate with private sector business experience, in turn derided Livingstone's record in office since 2000.

"I find it unbelievable that he is fighting this election on competence and ability to run London when he has presided over staggering waste and mismanagement," he said.

Both candidates played down suggestions the poll could have a wider impact on British politics.

Many observers see the race between Livingstone and Johnson - who are running neck and neck in the latest polls - as a key indicator for a national election that has to be held by 2010.

But Livingstone, 62, a member of Prime Minister Gordon Brown's Labour party, said he doubted the broader impact of the London-wide ballot, when up to 5 million Londoners could vote.

"I don't think there will be any great significance for national politics out of what happens in this city on May 1," he said.

"If this was a referendum on government it would be a very difficult election.

"I would be well behind in the polls - all the polls show it is neck and neck.

A poll at the weekend showed national support for the Labour government had slumped to 28 percent, with the Conservatives under David Cameron on 44 percent - enough to secure a parliamentary majority.

By contrast, recent polls in the capital have shown the leading candidates closely matched, with Johnson - an old school friend of Cameron's - marginally ahead.

Johnson said it was convenient for the media to assert that success in the London election was indispensable for either Brown or Cameron.

"What matters in this election is the future of London ... I don't think it is a necessary condition for a Conservative victory at the next election, because I think that is going to happen anyway."