Mr Foot, who has privately attracted the support of some Labour MPs, announced yesterday that he would stand to give Mr Livingstone's supporters a viable alternative in any contest.
Although he is unlikely to win, Mr Foot could cause serious damage to Labour's chances by attracting disgruntled electors away from the party's official candidate and splitting the total Labour vote.
His decision to run follows outrage within the London Labour Party at plans to set up a candidates' vetting panel that critics claim is designed specifically to prevent the former GLC leader from standing.
Mr Livingstone, who says he would not run as an independent himself, has threatened to organise a write-in campaign to get on Labour's shortlist if he is blocked by Labour's Millbank headquarters.
Mr Foot, a senior editor at Private Eye magazine and nephew of the former Labour leader Michael Foot, is likely to be backed by a network of activists who managed to get left-wing candidates on to the party's national executive committee this year.
The 61-year-old journalist said he would only stand if Labour stopped the Brent East MP from putting his name on any shortlist. "The main point I'm making is that it is an obvious scandal that the candidate wanted by Labour members is being stitched up by the Labour Party," he said.
"It would be a complete disgrace and outrage if they didn't let Ken stand. If that happens, then I am prepared to stand as a socialist candidate.
"I think I would be among the more credible candidates. I would use the power of the mayor's office to stand up for the poor of this city who have had such a hard time."
A Foot mayoralty would oppose privatisation of the Tube, increase public ownership of the buses and halt private finance schemes for schools and hospitals, he added.
Leonora Lloyd, a leading member of the executive of the London Labour Party until last year, said: "If Ken is blocked, there would be an awful lot of resentment in the Labour Party and a lot would unofficially throw their weight behind Paul Foot. There would be such a bad reaction that Labour members are likely to vote for anyone other than the official candidate."
Labour Party sources said yesterday that Mr Foot's membership of the Socialist Workers Party would deter most Labour supporters.
"If he's saying that most Ken supporters are so left-wing they would go for him then that proves our case that Ken is unsuitable," said one Millbank insider.Reuse content