Four men have been arrested on suspicion of murder after a prisoner was stabbed to death at HMP Wormwood Scrubs, Scotland Yard has said.
Two men aged 21, a 20-year-old and a 23-year-old man were detained in relation to the incident and remain in police custody.
All four were inmates at the west London prison, police said.
A man in his 20s was found with a number of knife injuries on Wednesday afternoon and was pronounced dead at the scene.
Police and paramedics were called to the prison at just before 3.20pm.
The man has not been formally identified and his family have been informed, police said.
A post-mortem examination will be held in due course.
Enquiries into the incident continue.