Johnson backs Miliband to be Labour leader
Deputy leader Harriet Harman has taken over as acting leader until the party chooses a successor to Gordon Brown.
Other possible contenders include former education secretary Ed Balls, Ed Miliband, veteran Jack Straw and backbencher Jon Cruddas.
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Hide AdMs Harman herself has signalled she does not intend to stand if she can retain her position of deputy leader.
After his dramatic resignation, Mr Brown and his family arrived at Edinburgh Airport late last night on their way back to their home in North Queensferry.
Reports claim he will now stand down as an MP and quit politics altogether. Friends said they expected him to concentrate on charity work.