A custody officer employed by G4S has resigned after a prisoner was found collapsed in his cell.
The 30-year-old suspect was meant to be checked on every 30 minutes while at a police station in Swansea, south west Wales. The G4S employee wrote in custody records that the suspect had been checked on but CCTV footage did not show the visit, the Independent Police Complaints Commission (IPCC) said.
The man, from Sketty, Swansea, was taken to hospital where he recovered after treatment.
An investigation into the so-called "near miss" has now been launched by the IPCC.
IPCC Commissioner for Wales Tom Davies said: "Fortunately, this man was found in time and has now recovered. Medical staff had advised that this particular man was to be visited at 30-minute intervals because of concerns about his health.
"There has been a lot of work and policies aimed at making detention safer and our investigation will examine what happened in this case and what specific training custody staff are given."
The suspect was arrested and taken into custody at about 3am on July 10. He was found collapsed at 4am and an ambulance was called by custody staff at Swansea Central Police Station.
He was discharged from hospital later that evening and also released from police custody on the advice of the Crown Prosecution Service. The custody officer later resigned, the IPCC said.
A G4S spokesman said the custody officer was guilty of "gross misconduct".
He said: "G4S has voluntarily signed up to the Independent Police Complaints Commission's code of conduct and we will cooperate fully with their investigation into this particular incident.
"G4S suspended a member of staff immediately after the incident. A subsequent internal investigation found that the individual failed to check on the welfare of detainees, which amounts to gross misconduct.
"The officer concerned is no longer an employee of G4S. We take the welfare of those in our care extremely seriously, and expect all our employees to observe the highest levels of professionalism at all times.
"Where there are instances of our employees falling below these high standards, then appropriate action is taken."