In the past few minutes, two Labour MPs - Margaret Hodge and Ann Coffey - have submitted a motion of no confidence in Jeremy Corbyn.
This is not a surprise given how angry the parliamentary party was after last night’s referendum results in its heartlands, but it now depends on whether their parliamentary colleagues have enough confidence to speak in its favour at a discussion at the parliamentary Labour party meeting on Monday, and then to back it in a secret ballot of the parliamentary party the following day.
It is not yet clear whether the rebels, who have been preparing for this for a while, yet have the numbers. And even if they do, Corbyn's side believe he would automatically make the ballot paper in a new leadership contest.
UPDATE, 12.40: The rebels confirm to me that they believe they have the numbers for this motion to be passed 'overwhelmingly'.
There is now a great deal of pressure on the Labour Shadow Cabinet. Will a bulk of its members resign en masse? Rebels are particularly keen to see movement from Andy Burnham, as the only former leadership contender serving in Corbyn's top team. One key plotter says:
'A lot hinges on Andy. He cannot just stay quiet because of the Manchester Mayoral nomination. Frankly, there may not be a Labour party to have a nomination from unless we can make the right judgements now of which who is our leader is one important one.'