London's Green Party voters are being asked to back Labour in the second preference slot at May's mayoral elections.
The mayoral election uses the Supplementary Vote system, under which voters rank their choices of candidates.
Second preferences come into play only if no candidate receives 50 per cent of the vote in the first round. In that case, second preferences are redistributed to the top two candidates from the first round.
Polling so far has showed Boris Johnson, the incumbent, and Ken Livingstone as neck and neck. The move would consolidate support for Mr Livingstone, who has faced heavy criticism this week after Jewish Labour voters alleged that he told them Jews would not back Labour because they were wealthy.
Jenny Jones, the Green Party's mayoral candidate, became a member of the London Assembly in 2000. She served as Mr Livingstone's deputy mayor between May 2003 and June 2004.
A poll this week by YouGov, conducted to mark the first day of the official mayoral election campaign, put Mr Johnson ahead, backed by 54 per cent of voters if the election were a two-horse race.