Labour MPs will today vote in a motion of no-confidence against party leader Jeremy Corbyn today following the resignation of around 40 members of his front bench in two days.
As many as 150 MPs are expected to vote to oust Corbyn, who has vowed to stay on in the top job.
"Don't let the people who wish us ill divide us," he told a crowd of his supporters at a rally in London's Parliament Square on Monday night, without directly referring to the leadership crisis.
He has reshuffled members of his existing shadow cabinet and threatened to recruit 100,000 hard-Left supporters to cling on to power.
Monday evening's rally followed an "extraordinary" Labour Party meeting that saw tearful MPs begging the party leader to resign.
On Tuesday evening MPs will vote on the no-confidence motion in a secret ballot.
The motion is not binding but will add to the pressure on the Islington North MP.
Corbyn's aides said critics who want to remove him from his position should launch a formal challenge, which they are confident he would win.
The Labour-supporting Daily Mirror joined the calls for him to quit "for the sake of his party and country."
It said filling dozens of vacancies with "inexperienced lightweights" was like "using a toothpick to prop up a collapsing skyscraper" and suggested Mr Corbyn's attempts at "clinging to power increasingly looks like an act of vanity".
Former shadow business secretary Angela Eagle is among those tipped as a possible challenger to Corbyn.