Frank Dobson has narrowly won his bitter battle with left-winger Ken Livingstone to be Labour's candidate for London mayor.

But minutes after the former health secretary squeezed home, Mr Livingstone branded the result "tainted" and urged Mr Dobson to stand down.

Mr Dobson polled 51.53% of the overall electoral college to Mr Livingstone's 48.47% - but while the Labour leadership's choice won the support of most MPs, Mr Livingstone won a clear majority among party members and those unions which balloted their members.

Speculation is growing that Mr Livingstone, furious at the way Labour handled the selection process, could now stand as an independent in the mayoral election.

At what should have been a celebratory press conference, Mr Dobson tried to laugh off Mr Livingstone's comments, saying he was "delighted" with the result.

And he urged Mr Livingstone to stick by his word and not stand as an independent candidate in the mayoral poll - going further by offering him a role in a Labour administration if Mr Dobson wins the mayoral election.

But he immediately faced a barrage of suggestions that he was not the legitimate choice for mayor of most Labour members.

In the members' section of the voting Mr Livingstone won 59.9% of the votes compared to Mr Dobson's 40.1%.

Second ballot

The vote had gone to a second ballot, with second preferences from those backing the other candidate, Glenda Jackson, proving crucial to the final result.

Although Mr Livingstone urged support for the Labour Party, the widespread criticism of the electoral process could hamper the party's chances of victory in the 4 May poll.

Mr Livingstone said outside his home: "In the interests of uniting the Labour Party I hope Frank Dobson will consider his position over the next few days."

Refusing to be drawn on whether he would stand as an independent in the mayoral elections, he said the result was based on the "theft" of eight per cent of the electoral college.

He said there was now a "serious question mark" over whether Mr Dobson could beat Tory candidate Steve Norris.

Meanwhile, Mr Dobson insisted he had won the Labour contest squarely, if not fairly.

"I got 40% of the individual membership when the original vote was taken. I have not been responsible for the introduction of this process but like Ken Livingstone and Glenda Jackson, I undertook before we knew the outcome to stand by the result, and I expect to other candidates to do likewise," he said.

Ms Jackson said that she felt she had achieved a "respectable result" and expected the Labour movement to unite to ensure the party provided London with its first directly-elected mayor.

The row over the Labour mayoral contest centres on the electoral college used.

Plans to hold a straightforward ballot of London party members were abandoned in favour of the college, with critics claiming it was set up to maximise the Mr Dobson's chances.

Voting in the contest was split between unions, party members in London and London MPs, MEPs and candidates for the Greater London Authority.

Mr Livingstone won more support than Mr Dobson among the unions and party members, while Mr Dobson won overwhelming support among Labour MPs, MEPs and GLA candidates.

The former health secretary also benefited from the block vote of the AEEU engineering union, which did not ballot its members.

But there was just enough support from party members for Mr Dobson for him to win across the college as a whole.

The results before Ms Jackson was ruled out of the contest saw Mr Dobson receive 49.569%, Mr Livingstone 46.009% and Ms Jackson 4.421%.

Mr Dobson will now prepare to battle Conservative candidate Steve Norris and Liberal Democrat candidate Susan Kramer in the May election.

Following the announcement of the result, bookmakers William Hill made Mr Dobson 4-11 favourite to become mayor, and offered 3-1 Mr Livingstone, 4-1 Mr Norris and 100-1 Susan Kramer.