An inmate has been murdered at Coldingley Prison in Woking, Surrey Police said.
Coldingley is a category C training prison, where inmates are considered unlikely to make a determined escape attempt but cannot be trusted in open conditions.
It has capacity for around 500 prisoners in five wings, where inmates are mainly housed in single cells, and is focused on the "resettlement of prisoners".
Steve Gillan, general secretary of the Prison Officers Association, said: "We do not comment on ongoing police investigation, but generally there has been an increase in violence, and indeed homicides, in our prisons.
"We believe that is due to the lack of prison officers – some 7,000 have been made redundant since 2010.
"Our prisons are a more violent place than they have been. It is tragic that someone has lost their life."
The Prison Service named the dead man as 25-year-old Madala Washington.
A spokesman said: "HMP Coldingley prisoner Madala Washington died in prison on Friday April 1.
"As with all deaths in custody there will be an investigation by the independent Prisons and Probation Ombudsman."
Surrey Police said it was investigating following the man's death at the prison in Bisley.
A spokesman said: "Officers attended the address shortly after 1pm today following reports of a serious assault on an inmate at the prison.
"Detectives are carrying out urgent inquiries to establish the circumstances surrounding the incident."
The death comes against a backdrop of concerns about rising cases of violence in UK prisons.
There were 257 deaths in custody last year including eight homicides, more than in any other year since records started in 1978.