Labour Party members in London are being issued ballot papers to select the candidate who will run for the office of London mayor in 2012.
They must choose between former mayor Ken Livingstone and ex-Bethnal Green MP Oona King.
Two other hopefuls, former Enfield councillor Seton During and Emmanuel Okoro, who is an artist, did not make it on to the candidates' shortlist.
The results of the vote will be announced on 24 September.
Mr Livingstone, who held the post for eight years, was ousted by the current Conservative mayor Boris Johnson in May 2008.
Ms King lost her Bethnal Green seat in 2005 to Respect's George Galloway.
The mayoral candidate will be picked by an electoral college, made up half-and-half of votes from London party members and members of affiliated organisations.
The ballot for the mayoral candidate opens on the same day as Labour Party members start voting for a new leader.
Five MPs - backbencher Diane Abbott and former cabinet ministers David and Ed Miliband, Ed Balls and Andy Burnham - are contesting to succeed Gordon Brown.
The results of the leadership race will be announced ahead of the party's annual conference and a day after the results of the 2012 mayoral candidate.
Mr Livingstone and Ms King will discuss the mayoral selection, alongside a debate featuring the Labour leadership candidates at Camden Town Hall, north London, on Friday.