The senior official investigating claims of coronavirus rule-breaking parties in Downing Street must be given access to Boris Johnson if required, opposition parties said, after it was alleged he attended a drinks party during the first lockdown.

       Downing Street declined to deny a report in the Sunday Times that alleged the Prime Minister attended the event in the No 10 garden with wife Carrie in May 2020.

       The newspaper cited three sources stating Mr Johnson’s principal private secretary, Martin Reynolds, emailed officials with an invite adding “BYOB”, meaning bring your own bottle, or booze.

       Senior civil servant Sue Gray is investigating the May 20 event as part of her inquiry into numerous allegations of rule-breaking events being held in No 10 during the coronavirus pandemic.

       Boris Johnson with his wife Carrie (Jacob King/PA)

       (PA Wire)

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       Deputy Labour leader, Angela Rayner said: “It is vital that Sue Gray has all the information and access she needs to carry out a full and fair investigation.

       “While the terms of reference for the new investigation have not been published, it is paramount that Sue Gray is given the same access – including if she needs to interview the Prime Minister.”

       Liberal Democrat MP Alistair Carmichael added: “The Government should not skirt around the issue on this. If they are serious about healing public trust, the head of the No 10 Christmas Party inquiry Sue Gray should personally interview the Prime Minister.”

       Allegations of that gathering, said to have been attended by 40 people, emerged this week when Dominic Cummings said he had warned at the time the “socially distanced drinks” would likely be against the rules and “should not happen”.

       But the front page report suggesting Mr Johnson himself was present will add to the seriousness of the claims.

       The Prime Minister imposed England’s first lockdown to combat Covid-19 in March 2020 and it was not until June 1 that groups of up to six people were allowed to meet outdoors.

       Dominic Cummings in Downing Street (Jonathan Brady/PA)

       (PA Archive)

       Ms Gray, a senior Cabinet Office official, also expanded her investigation to the May 15 2020 garden gathering which was revealed by a leaked photo showing the Prime Minister and staff sat around a table with cheese and wine.

       Mr Cummings, who was present in the picture, insisted in a blog post on Friday that there was nothing “illegal or unethical” about that day, which he insisted was a staff meeting.

       But the Vote Leave veteran, who departed as the chief adviser in Downing Street in November 2020, wrote: “On Wednesday 20 May, the week after this photo, a senior No 10 official invited people to socially distanced drinks in the garden.”

       Mr Cummings said he and at least one other special adviser “said that this seemed to be against the rules and should not happen”, adding that he issued the warning “in writing so Sue Gray can dig up the original email”.

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       “We were ignored. I was ill and went home to bed early that afternoon but am told this event definitely happened,” he continued.

       No 10 and a spokeswoman for Mrs Johnson were both asked to comment on the latest claim.