Two staff members at Pentonville Prison in north London have died after showing symptoms of coronavirus, the Prison Officers' Association (POA) has said.
The men, Bovil Peter and Patrick Beckford, worked as support staff.
Both are thought to have been in their 60s but it is not known if they had any underlying health conditions, the POA said.
Chairman Mark Fairhurst said: "My thoughts and prayers are with everybody involved with these tragic deaths."
He added: "Two at the same prison is very concerning."
Mr Peter was described as "an experienced member of staff" working at operational support grade at the prison, who "died earlier this week due to Covid-19 symptoms".
Mr Fairhurst said of Mr Peter's death: "I just want to highlight the fact that this [coronavirus] puts us all at risk.
"We are on the front line doing a commendable job on behalf of society and he will be sadly missed by all his colleagues. We wish the best for his family and friends."
Staff at the prison are thought to have held a minute's silence after hearing their colleagues had died within days of each other earlier this week.
A Prison Service spokesman said: "Our deepest sympathies are with their loved ones and colleagues at this difficult time."