A Tory leadership challenge would be triggered if 15 per cent of Tory MPs write to the chairman of the powerful 1922 Committee saying they no longer have confidence in Mr Johnson.
There were 360 Tory MPs – before Christian Wakeford defected to Labour – meaning that 54 need to submit a letter to prompt a confidence vote.
The 1922 Committee keeps the number who have submitted letters a closely guarded secret, but MPs are free to make public if they have done so.