Sadiq Khan has confirmed that he will quit as an MP if he becomes London mayor next year.
Khan insisted that being mayor was a "full-time job" and that he would quit as MP for Tooting, south London, if he won. He dismissed claims in the New Statesman that he had told supporters he would serve as an MP until 2020 no matter what.
Local Labour members fear that Tooting will fall to the Conservatives if Khan, a popular local MP for 10 years, stands down and forces a by-election. Tory candidate Dan Watkins fell short by just 2,842 votes in May's general election.
In an interview with BuzzFeed News, Khan said: "It's a long road between now and 2016 but I think mayor is a full-time job, so I think if I was given the privilege and honour of being mayor I don't think I could do another job."
Pressed on claims he had vowed to remain MP, he added: "It was reported by foes that that's what I'd said. For the avoidance of doubt, I believe mayor of London is a full-time job."
Aides said he would stand down "at the earliest opportunity" if he won the mayoralty, suggesting that it might not be immediate. Last year Khan branded Boris Johnson a "lame duck mayor" for choosing to stand as a Tory MP and juggle both jobs until 2016.
Tory MP Zac Goldsmith has announced that he will write to all his constituents in Richmond Park to ask for permission to run as mayor, paying the £50,000 cost of the ballot out of his own pocket. But Khan said he wasn't able to do the same.
"If you're a billionaire you can do many things," Khan told BuzzFeed News. "What I will do is make sure I will do all I possibly can to persuade other Londoners to support me in this election."
He laughed off allegations of a "trade union stitch-up" to make it easier for him to become Labour's mayoral candidate. "There is a primary," he said. "Every Londoner's vote counts equally, whether you are a Labour party member, a supporter, a nurse, a teacher, or a member of a trade union."
Khan also hit out at rivals Tessa Jowell and David Lammy for suggesting he was too close to Labour's failed leader Ed Miliband. Khan served as Miliband's campaign manager in the 2010 leadership contest and remains a close ally.
"Tessa Jowell served in Tony Blair's cabinet and I appreciate she's 100% loyal to Tony Blair, and David Lammy of course was promoted and pushed by Tony Blair lots," he said bluntly. "If they want to criticise me, I want to have a positive campaign. During 13 years in government Labour did some very good things and some things we got wrong. I think we need the humility to learn the right lessons but also to bounce back."
Asked whether Ed Miliband was backing him to be mayor, he said: "I don't think he's backing anyone. I've not spoken to him about my candidature."