Baroness Uddin, an "independent" member of the 13-strong London Selection Board, quit after intense pressure from Millbank officials. The peer, who was a Labour councillor before her elevation to the Lords, surprised observers last week when she criticised Mr Livingstone's record at the GLC.
As a member of the board, she was supposed to maintain a strict neutrality on the qualities of each of the possible candidates. Mr Livingtone said her comments were akin to a jury member deciding on a person's guilt before a trial. Labour MPs feared her comments would strengthen Mr Livingstone's case should legal action follow a decision to prevent his standing.
The board meets next week to interview the hopefuls before drawing up a shortlist.
Commenting on Lady Uddin's resignation, a party spokesman said: "She has told the Greater London Labour Party that she wishes to be free to contribute fully to the debate on which of the Labour hopefuls should be selected."
Mr Livingstone said: "I just hope that they allow me to nominate her replacement because the others are all against me too. One down, 12 to go."Reuse content