Dominic Raab has said it would have been a breach of Covid restrictions at the time if a party was held in No.10 Downing Street in the run up to last Christmas.
Boris Johnson has not denied reports that his staff held a party on December 18 last year when London was under Tier 3 restrictions but has insisted that no rules were broken.
Raab, the justice secretary and deputy prime minister, said that if reports based on “unsubstantiated claims all on the basis of anonymous sources” turned out to be true 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.”
The Metropolitan Police have said they are considering complaints from Labour MPs that No.10 staff held Christmas parties.
In a statement, the police said that while it did not routinely investigate “retrospective” breaches of the Covid regulations, it was considering the correspondence it had received.
On Wednesday, the Daily Mirror reported the prime minister gave a speech at a packed leaving-do for a senior aide last November when the country was in the midst of the second lockdown.
The newspaper said members of his No.10 team then held their own festive party in the days before Christmas, while London was under Tier 3 restrictions.
In each case, the paper reported, there were 40 or 50 people crammed “cheek by jowl” into a medium-sized room.