The party's ruling National Executive Committee has rejected the one-member-one-vote system used by the Tories and the Lib Dems to select their candidates.

Frank Dobson: It's the system which was used in Scotland and Wales

Instead it has chosen an electoral college, similar to the one used to choose Labour leader Tony Blair and the party's leaders in the Scottish Parliament and the Welsh Assembly.

Mr Livingstone said it appeared that this selection process had been decided on in order to maximise the vote of his rival, former Health Secretary Frank Dobson.

"It looks like a particularly old Labour sort of fix," he said.

But the London Labour Party chairman Jim Fitzpatrick MP rejected the claim that the system was designed to "stitch up" the left-winger popular with the party rank and file.

Jim Fitzpatrick: 'Restoring democracy to London'

"We are restoring democracy to London. This is about getting the strongest Labour candidate who will win the election next year," he said.

He admitted the system was "confusing" for people outside the party even though it had been used to choose party leaders "for some years now".

Mr Dobson insisted the electoral college vote could favour Mr Livingstone.

Split vote

Under the system, one third of the votes in the college will be allocated to trade unions and socialist societies affiliated to the London Labour Party.

Ken Livingstone says party officials want to stop him

Another third goes to London MPs, MEPs and Greater London Authority candidates. The final third is granted to all members of the London Labour Party itself.

But before the candidates get to go forward to the electoral college they must first gain the approval of a 13-strong selection panel.

It is made up of five members of the London Labour Party board, four independent members and four from the NEC.

Labour had said it would use a one-member-one-vote ballot to choose its mayoral candidate but, according to party sources, "public sector workers should be allowed a say in who is chosen".

'A last-minute decision'

Ken Livingstone: Lets bring 1200 councillors into the vote

After the decision was revealed, Mr Livingstone said: "We have gone through the last two years with the Labour Party indicating it will be one-member-one-vote of London party members.

"This is a last-minute decision."

He said the system was the same one which had caused anger in Wales, when the grassroots favourite, Rhodri Morgan, failed to win the Labour leadership of the Welsh Assembly.

Frank Dobson quit the cabinet to stand for mayor

He lost to Tony Blair's favoured candidate, Alun Michael, who picked up the bulk of the support of the trade unions through their block votes.

Mr Livingstone said he hoped that the unions would ballot their members, rather than using block votes, and called for the selection to be widened to include London's 1,200 Labour councillors.

The former leader of the GLC added that Mr Dobson was the prime minister's choice for mayor, but said the announcement of the electoral system "nails the lie that I was going to be ruled out on some disloyalty grounds".

Mr Dobson, who officially launched his campaign to become mayor on Tuesday, responded to the NEC's decision by saying: "I would have thought it might give an advantage to Ken but I simply don't know, I have had no part in putting it together."

"I assume that they thought it was right to apply in London the approach they used in Scotland."