UPDATE: Prisoner charged over alleged assault

UPDATE: Police have charged a 19-year-old man over an alleged assault which left a Bunbury Regional Prison inmate fighting for his life.

Police allege the incident occurred at Bunbury Regional Prison on Saturday.

A 42-year-old man who received life threatening injuries remains in a critical but stable condition in hospital.

The 19-year-old man has been charged with grievous bodily harm and is due to appear via video link in Bunbury Magistrates Court on Friday.

EARLIER: South West detectives are investigating an alleged “serious assault” which occurred between two prisoners about 9.40am Saturday.

A 42-year-old man received life threatening injuries and was taken to Royal Perth Hospital where he remains in a critical but stable condition.

Police say a 19-year-old man is helping police with their investigation.