He is urging voters in the capital to choose Labour at the polls, saying the prospect of a Conservative government would be an "unmitigated disaster for London".
The move has been criticised as a "cop out" by the Tories, who have accused him of "cosying up" to the government.
" Only a Labour government can provide the basis for improved public services "
Mr Livingstone was expelled by Labour last year after standing as an independent against its official candidate Frank Dobson in the mayoral elections.
Mr Livingstone 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.
"Over the next few weeks I will be out campaigning for Labour candidates across London.
"Only a Labour government can provide the basis for improved public services, and only the election of a Labour government would permit the conditions for the future prosperity of the capital."
Mr Livingstone maintained that since his election he had had an "excellent" working relationship with ministers.
He said he would campaign alongside Labour candidate Andrew Dismore (Hendon), Tony McNulty (Harrow East), Diane Abbott (Hackney North) and Iain Coleman (Hammersmith and Fulham) at future dates.
The announcement was seized upon by his political opponents.
The Tory group leader on the Greater London Authority, Bob Neill, said: "This is a cynical cop out by Ken Livingstone. He is going back on his word - instead of fighting for Londoners he is cosying up to the government.
"Londoners gain nothing from this deal but Ken will get his Labour membership card back. This is not about doing the right thing by Londoners it is about Ken getting re-elected."
Despite pledging his support to the Labour Party, it said nothing had changed concerning Mr Livingstone's re-entry into the party.