Boris Johnson is a “serial liar” and should be ousted from office after the latest revelation about a bring-your-own-booze drinks party at Downing Street, a former Conservative attorney general has said.

Dominic Grieve accused the Prime Minister of a pattern of behaviour that is undermining trust, including by lying about potential rule-breaking gatherings at Number 10 during the first coronavirus lockdown.

An email from Mr Johnson’s principal private secretary invited more than 100 Downing Street staff to an evening gathering on May 20 2020 when rules meant the public could only meet one other person outside.

Multiple reports – including from his former adviser Dominic Cummings – have suggested Mr Johnson attended the event with his wife Carrie.

Mr Grieve, a barrister who served as David Cameron’s attorney general from 2010 to 2014, said that Tory MPs who are “very unhappy” with the current Prime Minister’s behaviour should move to replace him.

The difficulty we have here is that we have a Prime Minister who's effectively a serial liar

Asked about the latest allegation of the Prime Minister and Downing Street staff flouting Covid-19 rules, Mr Grieve said: “He ought to be in a lot of trouble because he’s told a series of untruths about these issues over a period of time and the latest evidence clearly suggests that the rules were broken.

“There may be mitigating circumstances for that, I don’t know. But it’s part of a pattern of behaviour by him which undermines trust. And because trust is undermined, it then becomes very difficult to accept anything he says on any topic whatsoever.”

Mr Grieve said he thinks the public now has “very little” trust in Mr Johnson and added: “The difficulty we have here is that we have a Prime Minister who’s effectively a serial liar.”

Speaking on the BBC’s Good Morning Scotland programme, he said: “My impression is that there are many MPs who are deeply unhappy about him, but the difficulty is that replacing a prime minister requires courage, it requires the upheaval that goes with it, and it requires the coordination necessary to get him removed.

“I think it’d be very desirable if he were removed.

“I think the Conservative Party would find that it has a much better future without him than it has with him at present.

“But whether that’s going to happen in the short-term future, I think is open to doubt. In the longer term, I think that he is going to have great difficulty recovering from these events.”

Mr Johnson has refused to answer questions about the latest allegation of Downing Street rule-breaking, instead claiming the public should wait for the conclusions of an investigation by senior civil servant Sue Gray – a line repeated on Tuesday morning by health minister Edward Agar.

But Mr Grieve said: “I think that ultimately you have to make a judgment about whether somebody is suitable to be a political leader and the prime minister in a democracy.

“I don’t think the current Prime Minister passes that test and I don’t think it requires conclusions from Sue Gray to tell one that.

“Even if he were to escape Sue Gray’s censure, I don’t think it said to make any difference to his long-term behaviour.”

He added: “The conclusion about the Prime Minister’s understanding of propriety and truth doesn’t require Sue Gray to report, because the pattern of his misbehaviour is now so consistent on a large range of issues over quite a long period of time.”

Meanwhile, former Scottish Conservative leader Ruth Davidson said the public were rightly furious after more evidence of alleged rule-breaking by Downing Street was revealed by ITV, writing on Twitter: “What tf (the f***) were any of these people thinking?”

Referencing Mr Agar’s argument that people should wait until the conclusion of the investigation, Baroness Davidson said: “This line won’t survive 48 hrs.

“Nobody needs an official to tell them if they were at a boozy shindig in their own garden. People are (rightly) furious. They sacrificed so much – visiting sick or grieving relatives, funerals.

“What tf were any of these people thinking?”