Angela Eagle will challenge Jeremy Corbyn for the Labour leadership on Thursday, the BBC understands.
It is believed the former shadow business secretary has the support of the 51 MPs needed to mount a challenge.
Ms Eagle was one of 20 members of Mr Corbyn’s shadow cabinet to quit since Sunday, which led to MPs passing a motion of no confidence in Mr Corbyn.
Mr Corbyn’s allies say they believe he will win an election and have urged it to take place as quickly as possible.
Earlier, deputy leader Tom Watson ruled himself out of a potential contest and said Mr Corbyn was refusing to stand down despite his position being untenable,.
Mr Watson said the Labour leader had rebuffed his calls to resign, leaving the party “in an impasse” which risked turning into an “existential crisis”.
Mr Corbyn has refused to “betray” party members who backed him by resigning and his backers say a formal leadership contest would be needed to depose him.