During the initial lockdown, the PM was accused of attending a 40-person party in the Downing Street garden.
A leaked email from his Principal Private Secretary Martin Reynolds appears to invite 100 staff members to the dinner on May 20, 2020.
A new poll finds that 66 percent of UK adults say it is time for Mr Johnson to step aside.
Founder of Phones4U, John Caudwell, lambasted the government's ‘arrogance and hypocrisy'.
The claims are ‘disastrous' for the party, he told the BBC.
Mr Johnson should resign if proven to have deceived Parliament, said Scottish Conservative leader Douglas Ross tonight.
Mr Ross acknowledged he was 'enraged' that the Tory leader has yet to deny attending the gathering.
‘I'm enraged. That, I believe, represents current sentiments in Scotland and the UK.'
‘Nothing would undermine what Sue Gray is trying to accomplish, for the Prime Minister to come out and answer a very basic question,' Mr Ross said. Is he at the party?
In any case, it would inform the public in some way about her probe.
‘The Prime Minister needs to speak yesterday night. This email was made public yesterday. We had all day.'
When asked if Mr Johnson had deceived parliament, Mr Ross responded, ‘I believe we'll have to see what he says.'
‘But I am certain that any member of parliament who willfully misleads parliament cannot continue. I've said before they must resign.'
Mr Johnson may be ‘unsurvivable', according to Nigel Mills, the Tory MP for Amber Valley in Derbyshire.
'Anyone who went there in a senior capacity and believed it was okay to attend the party while they were announcing on TV the same night the regulations that you couldn't have parties can't possibly be determining Covid policy,' he told BBC News. We've seen individuals resign for far less.
‘How can the Prime Minister survive having accepted resignations for significantly less if he attended a party?
‘He accepted his spokesperson's resignation (Allegra Stratton) for not attending the party but joked about it. That is simply untenable.'
‘I don't think we need an inquiry to find out if the PM was there. He knows he was there.
‘Just say what occurred. Whether he was there, he should apologise profusely and see if the country accepts it.'
On Twitter this afternoon, Tory MP Johnny Mercer apologised to an enraged constituent.
I missed my father's 80th birthday and my wife worked 60 hours a week at a hospital, so any thoughts for the people of Plymouth you represent?'
‘I'm sorry,' Mercer said. My colleagues who at least try to lead by example are not represented by this.
You and your wife are appreciated. I will always put you first.'
Meanwhile, hundreds of other enraged Britons have been forced to bury loved ones on the same day.
The Metropolitan Police stated they are in communication with the Cabinet Office, despite declining to examine previous official social gatherings.