Sadiq Khan will serve a full term as MP for Tooting if he becomes Mayor of London

The move will head off fears that Labour will lose the seat in a by-election, but the Tories may use it against Khan.

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Sadiq Khan has told members that he will remain as MP for Tooting until 2020 even if he becomes Mayor of London, multiple sources have told the New Statesman

Khan made the announcement to his local party  at their AGM, where he was also nominated as  Mayoral candidate without opposition.

The move will ease local fears that without Khan's considerable local popularity, the seat would fall to the Conservatives in a by-election. Khan will instead vacate the seat in 2020, when he will seek a second term as Mayor at the same time as Labour will challenge the Conservatives in the general election.

But it may cause the Tories to paint Khan as a part-time Mayor. However, the limited success of the Labour Party in doing the same to Boris Johnson - who in addition to being Mayor of London, was a columnist for the Daily Telegraph and is now the MP for Ruislip & Uxbridge - suggests that this will prove fruitless.

Stephen Bush is political editor of the New Statesman. His daily briefing, Morning Call, provides a quick and essential guide to domestic and global politics.