The small opposition Liberal Democrats have said they will table a no-confidence motion in Primer Minister Boris Johnson a day after he survived a vote by his own MPs.
The Lib Dems announced the move on Twitter on Tuesday morning, hours after the PM saw off an internal Conservative Party challenge by 211 votes to 148.
The scale of the backbench rebellion may have emboldened the fourth-largest party, which holds just 13 out of 650 seats in the House of Commons, to chance the gambit.
"Liberal Democrats are tabling a motion of no confidence in the Prime Minister, to give Parliament the chance to finally put an end to this nightmare," the Lib Dems tweeted. "Every Conservative MP with a shred of decency left must back our motion and finally give Boris Johnson the boot."
Tweet by the British Liberal Democratic Party announcing a no-confidence motion in Prime Minister Boris Johnson
The main opposition Labour Party had made no sign of supporting the Lib Dems' motion by late morning, although their deputy leader Angela Rayner earlier told BBC Breakfast that Labour would "consider all options" in its ongoing bid to claim Johnson's scalp.
"He has no confidence of his backbenchers, he has no confidence of any other political party and he has lost the will of the British people. So he should do the right thing and resign," Rayner demanded.
But the motion targeting Johnson, rather than the government as a whole, is purely symbolic.
While a vote of no-confidence in the government would force a snap general election — which polls suggesting would leave Labour the largest party in a rare hung Parliament — one against the PM carries no legal force.
Labour tabled a similar motion against then-prime minister Theresa May in December 2018, days after she survived a confidence vote over her handling of Brexit — by a similar margin to her successor Johnson's victory on Tuesday. But May's government simply refused to grant time in the legislative agenda to debate that motion, a decision backed by then-Commons speaker John Bercow.