Ken Livingstone could lose his job as London mayor if he returns to the Labour Party, a poll has suggested.

The Populus survey, conducted for Conservative mayoral candidate Steven Norris, found that 23 per cent of Livingstone's supporters would be "less likely" to vote for him if he stood as the official Labour candidate.

When all other candidates are taken out, support for Livingstone stands at 47 per cent with 38 per cent backing the Conservatives.

"It must be an extraordinary piece of electoral history that a candidate is making a conscious decision to reduce his vote, because Ken can be in no doubt that is the price he'll pay for rejoining Labour," said Norris.

The survey comes as a Labour committee looks set to meet this week, to decide whether the mayor should be readmitted to the party.

He was expelled for five years in 2000 for standing against the then-official candidate, Frank Dobson.

Party insiders are said to fear that the official candidate this time, Nicky Gavron, could come fourth or even worse.

Gavron is said to be ready to concede the candidacy to Livingstone, in exchange for greater powers as his deputy mayor.

On Sunday former deputy leader Lord Hattersley warned the prime minister that it would be wrong to readmit the mayor to the party.

He attacked the prime minister, who is believed to have sanctioned Livingstone's return, for not "standing by the principle" of the Labour rulebook.

"Tony Blair wants to be on the side of the winning candidate in the mayoral election and I'm not sure that's a good way for a party leader to behave," he told GMTV.

"Sometimes a party leader has to say, win or lose, 'I'm standing by the principle' and I think that the principle of not standing against the Labour Party in elections is very important one."

But he admitted that the scene was set for Livingstone's early return to the party.

"I suspect it will happen with a minimum amount of trouble, and Ken will walk in as leader again as he would have done anyway but it will be another indication that Tony Blair isn't always rigidly adherent to principle," he said.