Conservative MPs will decide whether to remove Boris Johnson from office later, amid a backlash to the Partygate scandal.
The secret ballot has been triggered by scores of Tory MPs writing to party bosses calling on the PM to quit.
Anger among backbenchers has been rising since Sue Gray published her official report into lockdown parties in No 10 last month.
Cabinet ministers have been rallying round Mr Johnson ahead of the vote.
At least 180 Tory MPs need to vote against him to oust him from office, and a vote which passed that threshold would trigger a contest to replace him as Tory leader and PM.
But if a majority of them express their confidence in Mr Johnson's leadership, then he will be safe from another challenge for a year.