Hospital bosses have said the Prime Minister was "formally advised and reminded" of hospital rules on the day he was pictured not wearing a mask while greeting staff in a corridor.
Boris Johnson visited Hexham General Hospital in Northumberland on Monday morning - hours before MPs debated parliamentary standards.
Photographs showed him meeting masked nurses at the hospital, talking to them and bumping elbows with them, despite not having his face covered.
Soon after that photo opportunity, the Prime Minister put on a mask, which had been handed to him by an aide.
Shadow Health Secretary Jonathan Ashworth was among those critical of Mr Johnson, calling the incident "ridiculous".
Deputy Prime Minister Dominic Raab defended the Prime Minister.
Bosses at Northumbria Healthcare NHS Foundation Trust have now responded to members of the public asking why Mr Johnson was not wearing a mask.
They said: "In response to your communication about the coverage you have seen of the Prime Minister pictured without wearing a face mask at Hexham General Hospital, the Trust would like to assure you that infection prevention and control has always been and continues to be of utmost priority in all of our hospitals and in all other Trust sites.
"These rules include information about mask-wearing and other infection prevention and control measures, such as hand-washing, tucking away loose clothing (ties), and having rolled-up sleeves.
"After the Prime Minister left a welcome meeting, he walked along a mezzanine corridor, for a very short period of time, without a mask," they continued.
"This brief moment was captured on camera. As soon as this was identified he was given a mask and he put it on.
"The Prime Minister did wear a mask for the majority of the visit. He didn't have a mask on when he did his media interviews, but this was with agreement with everyone else in the room, who did wear a mask.
"We ask that everyone visiting our hospital sites continue to follow our infection control and prevention rules.
Asked on Monday 15 November to explain why he was pictured not wearing a mask both at Hexham and at COP26, Boris Johnson said:
"I wear a mask wherever the rules say that I should and I ask everyone else to do the same.