A vote of confidence in Boris Johnson will be held by Conservative MPs from 18:00 to 20:00 BST on Monday.
If he loses, he will be forced to stand down as prime minister.
The vote was triggered after at least 54 MPs of the prime minister’s governing party sent letters to Sir Graham Brady, the chairman of the 1922 committee of backbench MPs. He said he notified the PM on Sunday.
BBC political editor Chris Mason said Partygate had pushed many Conservative MPs ”over the line.”