Mr Ken Livingstone said last night he will campaign in support of the party which expelled him a year ago.
In a move criticised as a "cop out" by his Tory opponents, the London mayor urged voters in the capital to choose Labour at the polls.
Mr Livingstone, who was expelled by Labour last year after standing as an independent in the mayoral elections, will begin campaigning on Friday.
He said: "It is clear that the Labour government, despite all my political conflicts with it, offers the best conditions for the functioning of devolution in the capital."
However, Mr Livingstone's announcement appeared to do little to thaw his relationship with Labour. Asked whether the move would speed Mr Livingstone's re-entry into the party, a spokesman replied: "Nothing has changed."