Senior Tory politician William Wragg has called on the Prime Minister to resign after he attended a "bring your own booze" party at No 10 during England's first lockdown.
The Hazel Grove MP, who is chairman of the Public Affairs and Constitutional Affairs Committee, said he was "particularly concerned as a conservative MP" with events and an apology from Boris Johnson during Prime Minister's Questions had not reassured him.
Mr Johnson has lost the support of many within his own party following his apology for the gathering in the garden of the Downing Street residence in May 2020.
Speaking on BBC Radio 4's PM programme he said: "The Prime Minister's position is untenable and I don't believe it should be left to the findings of a civil servant to determine the future of the Prime Minister, and indeed, who governs this country."
He said he felt for his colleagues who are "frankly, worn out of defending what is invariably indefensible."
Southport MP, and Conservative backbencher, Damien Moore, says he was "disappointed at the clear lapse in judgement" of the Prime Minister.
In a statement on his social media he added: "I do not in any way support anyone blatantly breaking the Covid rules" but that he awaited the outcome of the internal inquiry by senior official Sue Gray into a series of alleged parties held during lockdown in No 10 and Whitehall.
He said: "Today, I was in the Chamber to hear the Prime Minister confirm that he attended the gathering on 20 May 2020.
"I am extremely disappointed at this clear lapse in judgement. I recognise his apology, and will await the outcome of the investigation being led by Sue Grey, which the Prime Minister has rightly said he will take responsibility for. The Government has promised to update Parliament as developments occur, and I will continue to follow these closely.
"Let me be clear: I do not in any way support anyone blatantly breaking the Covid rules, which were put in place to protect us all, and which most of us followed throughout the pandemic.
"My unwavering commitment as Southport's Member of Parliament is to this town and to those who live here. I will always support those who follow the rules and keep us all safe."
The Prime Minister has insisted he believed it had been a "work event" and Downing Street said he had never been sent an email encouraging staff to bring a bottle and "make the most of the lovely weather".
He said an inquiry was examining the situation but accepted "there were things we simply did not get right and I must take responsibility".
Labour leader Sir Keir Starmer also called on the Prime Minister to resign.