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.
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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.