Britain's Deputy Prime Minister has said it would have been a clear breach of Covid rules at the time if Prime Minister Boris Johnson's staff held a party in No 10 Downing Street in the run-up to Christmas last year.

Mr Johnson has not denied reports that members of his Downing Street team staged a party on 18 December 2020, when London was under Tier 3 restrictions, but has insisted that no rules were broken.

However Deputy PM Dominic Raab, who is also the Justice Secretary, said he did not know the truth of the reports based on "unsubstantiated claims all on the basis of anonymous sources", but if they turned out to be correct, then there would have been a breach.

"Until there is something substantiated, until it is more than anonymous sources, I think we are chasing shadows," he told the BBC's The Andrew Marr Show.

"If there is a breach of the rules, there is a breach of the rules. But I don't know the full facts because I wasn't there.

"Of course, if there was a formal party held, of course that it something that is clearly contrary to the guidance.

"If anyone held a party that it is contrary to the rules, of course that is the wrong thing to do.

"If something unsubstantiated from anonymous sources actually materialised, then of course it would be wrong."

His intervention has been seen as increasing pressure on Mr Johnson to explain what exactly went on in Downing Street last year, something he has repeatedly refused to do.