The SNP’s Westminster leader has said that Dominic Cummings is a “key witness” into the Downing Street party probe and without him “the inquiry will not be worth the paper it is written on”.

       Boris Johnson’s one-time top aide accused his former boss of lying to Parliament over allegations of lockdown-breaching bashes in Downing Street, and insisted he told the Prime Minister to get a grip on the “madhouse” when warning him over one “drinks party”.

       Ian Blackford the SNP’s Westminster leader, has now said that Mr Cummings was a key witness and called on the former adviser to be interviewed as part of Sue Gray’s expanding probe into parties held Number 10 during England’s coronavirus lockdown.

       Ian Blackford, the SNP’s Westminster leader, has called on the Prime Ministers former adviser to be questioned as part of Sue Gray’s party probe (UK Parliament/PA)

       (PA Media)

       “The Prime Minister’s former chief adviser, who was in post at the time Boris Johnson broke the law, has said he is willing to testify on oath that the Prime Minister was warned about the Downing Street party, allowed it to go ahead anyway, attended it himself, and then lied to Parliament.

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       “Lying to Parliament would be a clear breach of the ministerial code alone, and would require the Prime Minister to resign immediately,” the MP for Ross, Skye and Lochaber said on Monday.

       “That makes Mr Cummings a key witness in Sue Gray’s inquiry.

       “Without his account, the inquiry will not be worth the paper it is written on.

       “The Prime Minister must now confirm that Sue Gray will interview Dominic Cummings as part of her investigation.

       “Mr Johnson must then come to Parliament and put on record whether he believes Mr Cummings is lying, or whether he will finally admit that he has repeatedly lied to Parliament and the people himself.”

       The Prime Minister’s chief adviser said Mr Johnson “waved it aside” when he raised concerns over principal private secretary Martin Reynolds inviting more than 100 people to a “bring your own booze” event in the No 10 garden on May 20 2020.

       Mr Cummings said, regarding that day alone, “never mind the string of other events”, the Prime Minister “lied to Parliament about parties” by insisting he had been assured no events had taken place that would have broken coronavirus rules.

       “Not only me but other eyewitnesses who discussed this at the time would swear under oath this is what happened,” he said.

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       Sonia Khan, who was marched out of Downing Street by police officers on the orders of Mr Cummings when she worked as then chancellor Sajid Javid’s adviser, said a long-standing drinking culture saw staff start boozing at lunch and waking up there in the same clothes the next day.

       The Prime Minister will reportedly order a booze ban in No 10 under the so-called Operation Red Meat policies designed to save his leadership from demands for his resignation.