According to sources within Labour, a resolution will be introduced today with the goal of forcing a Commons debate over whether the PM still has a majority.
After Sir Keir insisted that it is untenable for Mr. Johnson to remain in his position while a replacement is picked, the action is meant to increase the pressure on Tory MPs.
Mr. Johnson would be expected to step down right away if he loses.
Nevertheless, the attempt is bound to failure because, despite the coup that saw the PM declare his resignation, the government still has a sizable majority.
Conventionally, the administration schedules a confidence vote at the earliest opportunity, which would be tomorrow, when the official Opposition asks one.
This procedure is distinct from the Tory confidence vote Mr. Johnson faced last month, when the premier defeated the first attempt to unseat him.
Sir Keir taunted, “Bring it on,” as he reiterated his call for a general election to remove the “zombie” Tory government.
In response to Boris Johnson’s declaration on Thursday that he will make way for a new occupant of No. 10, the Labour leader charged that Johnson was “squatting in Downing Street.”
Sir Keir urged an immediate general election, claiming that the nation needs a “new start.”