The last time a Conservative prime minister was under this much pressure from eurosceptics, they brought a leadership challenge on themselves.
In his famous “put up or shut up” challenge to the “bastards” that were making his premiership a misery, John Major resigned as party leader to draw out eurosceptic plotters. He emphatically beat John Redwood in a ballot of MPs by more than two to one.
Under the current party rules, that option isn’t open to Prime Minister Theresa May, who must wait to see whether Brexiteers can gather enough letters expressing no confidence in her leadership.
To trigger a vote of no confidence, 15 per cent of the Commons parliamentary group must write to the chairman of the backbench 1922 Committee, Graham Brady. Letters can come with conditions attached, but they have no shelf life, so even if the threshold is reached, Mr Brady will check all the senders are sure. If 48 letters are collected, a confidence vote would be organised for a day parliament was sitting.
The vote is decided by simple majority, and while Downing Street says Mrs May would fight on even after a narrow win, she would be seriously weakened. However, Brexiteers can’t afford to swing and miss: another confidence vote cannot be called for a year.
If May loses, leadership candidates will be nominated and depending on the number, whittled down in bi-weekly ballots of MPs until the last two are voted on by the Conservative Party membership.