Londoners will be able to vote in American presidential-style primaries to choose Labour's next candidate for mayor.
Party members and people who register as Labour supporters can have their say.
The change announced today by Ed Miliband reduces the impact of trade unions on the selection of candidates.
The details now from our Political Correspondent Simon Harris
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The film had formed part of a commemoration to the Choucair family, three generations of whom died in the fire.
John Nolan, who was found with cigarettes and two lighters, suffered burns on 65% of his body after being alight for around six minutes.
Known as Pily she died in January, seven months after the disaster, having suffered a stroke.