At least 145 British lawmakers from Prime Minister Boris Johnson's Conservative Party had publicly indicated support for him ahead of a confidence vote over his leadership on Monday, a Reuters tally showed.
A total of 180 votes would be enough to ensure he survives the leadership challenge. The ballot will be secret and there is no guarantee lawmakers will vote in line with their public statements.
The vote is due to commence at 6 p.m. (1700 GMT), with a result expected at 9 p.m.
Johnson, who scored a sweeping election victory in 2019, has been under increasing pressure after he and staff held alcohol-fuelled parties in his Downing Street office and residence when Britain was under strict lockdowns due to COVID-19.
Should he lose the vote, a leadership contest would follow to decide his replacement, which could take several weeks.
In what might concern Johnson's team, he said he was not aware of "any orchestrated campaign" to oust the prime minister, which suggests a more spontaneous rebellion than ones that have felled leaders in the past.
A spokesperson for Johnson's Downing Street office said the vote would "allow the government to draw a line and move on" and that the prime minister welcomed the opportunity to make his case to lawmakers.
(With inputs from agencies)
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