Calls from the Conservative benches for the Prime Minister to quit have grown steadily in recent weeks in the wake of 'partygate'.
A further three Tory MPs on Wednesday (February 2) called for Boris Johnson to stand aside.
A leadership contest can be triggered if 15 per cent of Conservative MPs write to the chair of the backbench 1922 Committee saying they no longer have confidence in Mr Johnson. There are currently 359 Conservative MPs, consequently a total of 54 letters would force a vote.