The MP for the Cities of London and Westminster has withdrawn her support for the Prime Minister.
In a letter to her constituents Ms Aiken said that she had been unhappy with Boris Johnson’s leadership for “some time”, and that she now believes he should step aside.
It comes in a day of drama in Westminster as at the time of writing 36 ministers and ministerial aides have resigned from government in the last 24 hours, the most notable being the now former Chancellor Rishi Sunak and Health Secretary Sajid Javid.
Ms Aiken, who had previously called on the Prime Minister to hold a vote of no confidence before winning one last month, said in her letter that MPs are bound by a contract of “trust and respect”.
She said: “I have been unhappy with the Prime Minister’s conduct for some time.
“Following the investigations into conduct in Downing Street during the lockdowns, it is clear that trust in both the Government, the Conservative Party and politics more widely has been seriously damaged.
“And, further to the mishandling of serious allegations made against the former Deputy Chief Whip, I strongly believe now is the time for a new Leader of the Conservative Party and that the Prime Minister should step down.”
She added that she had informed the chair of the influential 1922 backbench committee, Sir Graham Brady, of her views, and that in February she left her position as Vice Chair of the party.
Ms Aiken’s decision also came on the same day that MP for Kensington, Felicity Buchan, decided to resign as a Parliamentary Private Secretary in the Department of Business, Energy and Industrial Strategy.
Many backbench MPs are thought to have now written to Mr Brady to demand a rule change to 1922 rules which immunise the Prime Minister from any further votes of no confidence for 12 months.
Internal elections to the committee’s executive are due to go ahead next week, with some MPs running on a platform to allow a fresh leadership challenge.