A MURDER investigation has been launched following the death of an inmate at Cardiff prison.
Police have arrested a prisoner at the jail following the death of Darren Thomas, 45, who was found by staff in his cell at 3.25am on Thursday.
The Ministry of Justice (MoJ) said an ambulance was called but the prisoner was pronounced dead 35 minutes later.
South Wales Police officers at Cardiff Bay Police Station are now questioning a 22-year-old man.
A prison service spokesperson said: "HMP Cardiff prisoner Darren Thomas was found by staff in his cell at around 3.25am on Thursday 6 March. An ambulance attended and he was pronounced dead at 4am.
"This is now a matter for the police."
The coroner has been told.
Prisoners at category B HMP Cardiff are usually from south east Wales.