After several days of mass resignations - and numerous calls for Jeremy Corbyn to quit of his own accord - Labour MPs are expected to announce a formal challenge to his leadership today.
It comes after a motion of no confidence in him was supported by three quarters of the parliamentary party.
Two possible candidates are the party's deputy leader Tom Watson and ex-shadow business secretary Angela Eagle.
But Mr Corbyn has said he will not "betray" his supporters by resigning.