A prisoner at HMP Littlehey in Cambridgeshire is reportedly the first to have died from coronavirus.
The sex offender, 84, passed away on Sunday.
The news comes as the number of cases in prisons has risen to 19 across 10 different prisons in the UK.
They are a risk area as people are forced to live in close proximity.
An investigation into his death will now follow - normal for anyone who is in custody at the time.
As with all deaths in custody, there will be an independent investigation by the Prisons and Probation Ombudsman.
A Prison Service spokesperson told Mirror Online: "A 84-year-old prisoner at HMP Littlehey died in hospital on 22 March. Our thoughts are with his family at this time.
"As with all deaths in custody, there will be an independent investigation by the Prisons and Probation Ombudsman."
A source told The Sun: "There are a number of shared cells.
"It’s not clear if others, including inmates and prison officers, have been affected."
The prisoner who died is thought to have had underlying health conditions.
A source said he was confirmed to have been diagnosed with COVID-19 before his death.
HMP Littlehey is a Category C male sex offenders prison which holds up to 480 inmates.
UK prison visits cancelled
All visits to prisons in the UK have been cancelled, as many staff are reportedly in self-isolation.
As of lunchtime on Wednesday, 19 prisoners had tested positive for coronavirus across 10 different prisons, the Ministry of Justice said.
Some prison governors have called for the release of certain prisoners to prevent jails from becoming overwhelmed by the crisis.
Head of the Prison Governor's Association Andrea Albutt told The Guardian the release of some prisoners would ease pressure on the system, slowing the spread of the virus and preventing disorder.
She said: “Overcrowding, poor regime, reduced contact with family because we’re not having visits anymore – these are all things that can introduce instability.
“If we can take one of them away, if we can reduce the level of overcrowding in our prisons, it will help towards the coming months.”
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