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.
