Andy Burnham has won the Makerfield by-election with more than half of the vote, opening the door to challenge Sir Keir Starmer for the leadership.
In his victory speech on Friday, Burnham described his win as a "turning point" for the party, while he pledged not to use the constituency as a stepping stone.
However, Burnham has consistently polled as the most popular candidate for leadership of the Labour Party, and allies now believe his victory gives him a huge vote of confidence to challenge the increasingly unpopular Starmer.
But how would Burnham trigger a leadership contest, and how long would it take?
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