Ken Livingstone has been warned by Labour's leadership his days as a maverick are over.

Deputy prime minister John Prescott issued a tough message to London's mayor that if he wanted to rejoin the Labour party he would have to follow the party line.

Prescott, who has been fiercely opposed to moves to readmit Livingstone to the party, said on Sunday that any deal would have to include a pledge of loyalty when the mayor appears before an NEC panel on January 9.

"Ken has to recognise if you are a member of the Labour Party, you have to sign up for the rules," he said.

"In January, he will have to satisfy us - and presumably he will - that he will sign up to recognise the party's authority in determining policy. I presume if he wants to be a Labour Party member, he will want to sign up to our rules and I look forward to hearing him saying that, though I must admit he did say it once before ...''

The party suspended Livingstone in 2001 for five years for running as an independent against Frank Dobson.

But despite Prescott's tough talking, his selection is already looks a certainty following the decision by Nicky Gavron, Labour's candidate for the 2004 race, to stand down.

And Livingstone looks set to win the Capital's mayoral race, according to a new opinion poll.

Just days after moves were made by Labour to readmit him back into the party, research revealed he will beat his nearest rival next year by a massive 18 per cent.

The YouGov poll for the Evening Standard put Livingstone - who is still an independent candidate - on 49 per cent without backing from Labour.

His Conservative challenger Steve Norris faces an uphill struggle with 25 per cent while Liberal Democrat Simon Hughes trails on 16 per cent.

Nicky Gavron registered just five per cent.

Her decision to stand down this week followed weeks of negotiations and rumours but according to the poll any deal could actually cost Livingstone votes.

Moving from independent to Labour would see him lose five per cent but that still leaves him in an unassailable position.

The poll is likely to shift focus to Conservative candidate Norris, who has been heavily criticised for his directorship with rail contractors Jarvis, and the fact that he has just six months left to make an impact on Livingstone's lead.

The survey also had more bad news for Labour - its support in the Capital has fallen 10 per cent since 2001 to stand at 37 per cent, just one point ahead of the Conservatives, while the Lib Dems are on 23 per cent.