Jeremy Corbyn and John McDonnell have apologised over Labour's "catastrophic" defeat in Thursday's election, which saw them lose 59 seats.
Mr Corbyn said he was "sorry that we came up short", while Mr McDonnell told the BBC he "owns this disaster".
The leader and shadow chancellor said they would step down in the new year.
The race for their replacements has already begun, with Wigan MP Lisa Nandy saying for the first time she was "seriously thinking about" running.
Mr McDonnell said it would be up to Labour's National Executive Committee to decide the mechanics of the leadership election, but he expected it to take place in eight to 10 weeks' time.
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