G4S told wrong family of Mohamoud Ali's Parc Prison death
The family of a prisoner at Parc Prison in Bridgend was wrongly told he had died, BBC Wales understands.
Mohamoud Ali, 36, was found "unresponsive" in his cell by prison staff at about 07:00 GMT on Saturday, 1 February.
But security firm G4S has admitted officers informed the incorrect family and apologised.
Prison staff and paramedics failed to revive Mohamoud Ali and an investigation is under way.
A G4S spokesperson said: "The death of a prisoner in custody is an extremely distressing time for the family and we make every effort possible to ensure that next-of-kin are informed quickly and with the utmost sensitivity.
"What happened on Saturday is completely unacceptable and we have approached both the family and the prisoner so that they may receive a personal apology."