Health bosses have launched an urgent investigation after a group of medical staff met up for a team social against coronavirus rules – with some of them later becoming infected with Covid-19.

An internal memo to staff at Southmead Hospital in Bristol seen by the PA news agency, said two of them subsequently tested positive for the virus, with all involved then forced to self-isolate.

The hospital trust described the incident as “very disappointing” but said it did not believe patients had been put at risk.

Chris Burton, medical director of North Bristol NHS Trust, said in the memo to staff this week: “Unfortunately I have to draw your attention to a serious breach of the guidelines that are there to protect us all and our patients from transmission of Covid-19 virus.

“It is important that we all learn from this event to minimise risks to staff and patients in the future.

“A small number of staff members recently socialised together away from work, without adequate social distancing in place and in breach of national guidance.”

Dr Burton said the incident, classed as an official Covid-19 outbreak, had been reported to the relevant external public health and regulatory bodies, and that a full investigation is under way internally.

“All of the individuals involved are following the requirement for self-isolation,” he said.

“A full track and trace of any patient contact has taken place and there is no evidence of transmission to patients at this time.”

In a warning to staff, he added: “As medical director I wanted to write to you to reiterate the serious nature of this and the need to follow all Government guidelines, even when outside of work.

“This is a stark and serious example of the need to uphold the correct behaviour at all times.”

In a statement, Dr Burton added: “Protecting our patients is our utmost priority and we know that the vast majority of our staff are following social distancing guidance, so this incident that took place outside of work is very disappointing.”

The night out is believed to have taken place in the lead up to new lockdown restrictions for England coming into force on November 5.