Green voters should give their second preference votes to Ken Livingstone to prevent Boris Johnson winning the London mayoral election, members have decided.
The decision follows a question-and-answer session with the Labour candidate on Monday, in which Green party members interrogated him on his commitments for a new term in City Hall.
"Ken Livingstone is far from perfect, but we know from his last time as mayor that we can work with him to make positive changes in a way that would be impossible with either Boris Johnson or many other senior Labour politicians," Green party mayoral candidate Jenny Jones said.
"Should I not be counted among the top two candidates after the first round, then I want a mayor who will work with Green party assembly members to deliver on pay equality, less pollution and cheaper fares."
Greens did voice concerns about Mr Livingstone's record on road building, regulation of the financial sector and air pollution, however.
"What we are seeing is that as we get closer to the election a broadening alliance of people wants a fairer London," Mr Livingstone said.
"The Green endorsement for second preferences is a key building block to winning change on May 3rd.
"I am very pleased that the Green party has decided to encourage their supporters to cast their second preference votes for me."
Ms Jones, a member of the London Assembly, is fighting Liberal Democrat Brian Paddick, who is labouring under the unpopularity of his party at the national level, for third place in the election.