Scotland Yard today said they forced entry into the fourth-floor flat on Clayponds Lane in Brentford shorty before 1am this morning.
Officers found Kuha Raj Sithamparanathan, 42, suffering with stab wounds as well as the bodies of his wife Poorna Kaameshwari Sivaraj, 36, and their three-year-old son Kailash Kuha Raj.
Kuha was treated by paramedics but was pronounced dead at the scene.
Detective Chief Inspector Simon Harding, from the Met’s specialist crime command, said: ‘Although we are in the infancy of this investigation, it appears that Poorna and Kailash had been dead for some time.
‘This is a murder investigation and my team will work diligently to establish the sequence of events that led to their murders and the death of Kuha Raj Sithamparanathan.
‘Our initial enquiries have established that Poorna and Kailash had not been seen or heard from for some time, perhaps since around September 21.’
The Directorate of Professional Standards has made a referral to the Independent Office for Police Conduct as it is believed that Kuha fatally injured himself when officers entered the property, the force said.
The family’s Cockapoo dog, which they would often be seen walking, is also believed to have been killed.