A second inmate at the same prison - and the third in the UK overall - has died after contracting coronavirus.
The 77-year-old inmate at HMP Littlehey in Cambridgeshire died in hospital on Friday, the Ministry of Justice (MoJ) said. He had underlying health issues.
Fellow HMP Littlehey prisoner, the paedophile Edwin Hillier, 84, was the first serving British inmate to die with coronavirus, on 22 March.
The Prison Officers' Association said the virus "does not discriminate".
Its national chair Mark Fairhurst said that coronavirus "places everyone in our secure establishments at risk".
Lawyer Simon Creighton, who has inmates among his clients, told BBC Newsnight that prison was a very unsafe environment.
"If you think about how cruise ships have been seen as Petri dishes, prisons are about 1,000 times worse," he said on Wednesday.
"Some of the contact I've had with prisoners over the last week has been terrifying."
Mr Fairhurst pointed out that it was the older inmates who were most vulnerable.
"The two deaths at Littlehey demonstrate that an ageing prison population that have underlying health conditions are more at risk," he said.
"We would hope that wings at Littlehey can isolate their prisoners effectively and would insist that any interaction between staff and prisoners utilises full personal protective equipment.
"I would like to commend the brave frontline staff at Littlehey for the unavoidable risks they are enduring to care for the most vulnerable in our care."
A 66-year-old male inmate at HMP Manchester, who had been receiving end-of-life care for an underlying health condition, died in hospital on 26 March after contracting coronavirus.
As of 17:00 BST on Monday, 65 inmates had tested positive for coronavirus in 23 prisons, the MoJ said.
It has said it has "robust plans" in place to protect staff and inmates.