The Prime Minister has come under fire from the World Health Organisation’s special envoy for Covid-19 after being photographed without a face covering during a hospital visit.

Asked about pictures of Boris Johnson walking through Hexham General without a mask, Dr David Nabarro said: “We all need to be able to do what we’ve got to do, regardless of what politics we adopt.”

He told Sky News: “I’m not sitting on the fence on this one – where you’ve got large amounts of virus being transmitted, everybody should do everything to avoid either getting the virus or inadvertently passing it on.

“We know that wearing a face mask reduces the risk, we know that maintaining physical distance reduces the risk, we know that hygiene by regular hand-washing and coughing into your elbow reduces the risk.

“We should do it all and we should not rely on any one intervention like vaccination on its own.

“So, please, would every leader be wearing face masks, particularly when in indoor settings.

“This virus is unforgiving and we need to do everything possible to prevent it getting in between us and infecting us.”

Dr Nabarro also told the broadcaster: “Perhaps that’s the toughest thing about Covid, is that the responses get politicised – in some places if you wear a mask, you’re taught to belong to a particular political party.

“That’s the most regretful thing about the present situation; we all need to be able to do what we’ve got to do, regardless of what politics we adopt.”

The latest row came after Mr Johnson was pictured last week at the Cop26 climate change summit sitting alongside 95-year-old environmentalist and broadcaster Sir David Attenborough.

A No 10 spokesman earlier said of his visit to Hexham General Hospital: “The Prime Minister followed the Covid measures that were in place throughout his visit, including wearing a mask in all clinical areas. He also took a PCR test before the visit.

“The hospital trust has issued a statement making clear the PM followed its guidelines.”