POLICE were called by ambulance staff to a house in Ryefield Avenue, Hillingdon, at 12.53 this lunchtime following reports of an assault.
They found an unconscious man aged in his 40s receiving emergency treatment.
The man died at the scene shortly after 2pm.
Next of kin yet to be informed and the man has not been formally identified.
A man, aged in his 50s, has been arrested in connection with the incident and has been taken to a West London police station for questioning.
Inquiries are ongoing.
Officers from the Homicide and Major Crime Command and Hillingdon borough are investigating.
Anyone with information is urged to call police on 101 or Crimestoppers anonymously on 0800-555-111.
Road closures are in place in the locality.
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