Hospital bosses have spoken out after Boris Johnson was pictured speaking to staff without a face mask.
The prime minister chatted and bumped elbows with masked nurses at Hexham General Hospital in Northumberland on Monday morning.
Soon after the photo opportunity, Mr Johnson put a mask on after an aide handed him one.
Mr Johnson was then seen without a face covering during a television interview at the hospital.
Members of the public questioned why the prime minister was not wearing a mask when the government advice to help prevent the spread of COVID-19 is to wear one "in crowded and enclosed spaces where you come into contact with people you do not normally meet".
Bosses at Northumbria Healthcare NHS Foundation Trust, which runs the hospital, said everyone on the official visit was "formally advised and reminded" of the "very clear" mask-wearing rules and information was "included in the written programme".
They reassured the public that infection prevention and control "has always been and continues to be of utmost priority" in all its hospitals and sites.
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The bosses said the prime minister had left a welcome meeting and was walking along a corridor "for a very short period of time without a mask".
"This brief moment was captured on camera," they said.
"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."
The trust's bosses apologised "for any upset this has caused" and hoped their statement "helps to clarify the situation".
They added that the hospital rules include mask wearing and other infection prevention and control measures, such as hand-washing, tucking away loose clothing such as ties and having rolled-up sleeves.
Their statement came after Deputy Prime Minister Dominic Raab insisted on Tuesday that Mr Johnson had followed "all the protocols and procedures" while he was at the hospital.