The race to become mayor of London, with its implications for national politics and the 2012 Olympics, tightened on Thursday with polls putting the Conservative and Labour rivals neck-and-neck.

Boris Johnson, a boisterous ex-journalist turned Conservative member of parliament, leads Labour-backed rival Ken Livingstone by 51 percent to 49 percent according to a new Guardian ICM poll, a much narrower margin than just a week ago.

Incumbent mayor Livingstone, 62, and Johnson, 43, are bitter opponents not just in terms of ideology and party affiliation, but history and upbringing - Livingstone has working-class, left-wing roots, while Johnson was educated at Eton and Oxford.

Prime Minister Gordon Brown has reluctantly given his backing to Livingstone's campaign for a third term, despite some differences between the two, and Conservative Party leader David Cameron is actively promoting Johnson, an old school ally.

With an election due possibly as early as next May, both party leaders are hoping for a boost from the mayoral race. London's mayor enjoys the biggest electoral mandate in the country, with up to five million people expected to vote.

If Livingstone were to lose it could be a big blow to Brown, who has struggled to connect with voters since taking over from Tony Blair last June and has seen his party's fortunes take a turn for the worse, consistently lagging the Conservatives.

Cameron is hoping a Johnson win will add momentum to his bid to unseat Brown, while boosting the Conservatives' influence in London, a city that has traditionally been a Labour stronghold.

HIGH STAKES

At stake for Johnson and Livingstone is one of the most powerful political prizes in the land. As well as running an 11 billion-pound budget, the mayor will lead London up to its hosting of the Olympic Games in 2012, and be tasked with keeping it at the forefront of global finance.

The campaign has largely been a personality driven affair so far, but in the four weeks that remain before the vote, issues such as climate change, urban planning, youth crime, a much-maligned public transport system and affordable housing are expected to come to the fore.

Livingstone, a policy wonk who has run London since 2000 and is regarded even by his detractors as a determined manager, is hoping that he'll be able to edge Johnson if voters start focusing on the details of his environment and crime policies.

Johnson's advisers have so far tried to keep the campaign light, playing up their candidate's buffoonish, fun-loving personality and keeping him away from hardcore policy debates.

Behind Johnson and Livingstone in a campaign that is not short on colourful characters stand Brian Paddick, an openly gay former police chief who represents the Liberal Democrat party, and a 33-year-old member of the Green Party.

Johnson and Livingstone are far ahead and are widely expected to remain the front-runners, but the ballot includes a "second preference" vote which could prove key if the race does end up being as tight as predicted on the day.