Boris Johnson is under fresh pressure to quit as three more Tory MPs broke ranks in the wake of Sue Gray's scathing Partygate report.
Conservative MP John Baron said he did not believe Mr Johnson's claims that he was unaware of the scale of lockdown boozing in Downing Street laid bare in the dossier.
"Therefore, his repeated assurances in Parliament that there was no rule-breaking is simply not credible," he said.
In a statement on his website, the Basildon and Billericay MP said: "Having always said I would consider all the available evidence before deciding, I'm afraid the Prime Minister no longer enjoys my support - I can no longer give him the benefit of the doubt."
The Brexiteer backbencher tore into the "shameful pattern of misbehaviour during the pandemic as the rest of us kept to the Covid regulations" in No10 and Whitehall, adding: "Those responsible for setting the rules have a special duty to adhere to them."
Conservative David Simmonds, who was elected in the 2019 Tory landslide, also added his voice to those urging Mr Johnson to quit.
The Ruislip, Northwood and Pinner MP, whose constituency borders the PM's Uxbridge seat, said: "It is clear that while the government and our policies enjoy the confidence of the public, the Prime Minister does not. Accordingly, it is time for him to step down."
It comes after York Outer MP Julian Sturdy said the PM should go as Ms Gray's report "clearly shows the Prime Minister has presided over a widespread culture of disregard for the coronavirus regulations".
He added: "While I thought it important to wait for the conclusion of the Metropolitan Police investigation and the publication of the Sue Gray report, I am now unable to give the Prime Minister the benefit of the doubt and feel it is in the public interest for him to resign."
Mr Sturdy's statement landed as the Prime Minister gave a speech to the 1922 Committee of Tory backbenchers on Wednesday night to try win back the trust of his party.
Tories can mount a leadership challenge if 54 MPs send letters of no confidence in the Prime Minister to 1922 Committee chairman Sir Graham Brady.
The tally of letters is a closely guarded secret, and only Sir Graham knows when the threshold is met.
The PM's chief of staff Steve Barclay earlier denied that Boris Johnson told lies about Partygate.
Asked by Sky News if the Prime Minister has honour, Mr Barclay said: "Yes he does."
Asked if Mr Johnson tells lies, he said: "No he doesn't."
Mr Barclay said: "You asked my opinion, I work very closely with him. He is focused on our response to Ukraine. He is focused on the huge challenge economically for families, for your viewers, in terms of the cost of energy, the cost of food, he is getting on with the job."
He added: "He has been getting the big calls right but he accepted, in terms of some of these incidents, that there were lessons to be learned."
