A woman and two children have been found dead at a house, police said.
The 34-year-old woman and two girls, aged nine and four, were discovered at a property in Midcroft, Ruislip, at 6.40pm on Saturday night.
Scotland Yard said detectives from the Homicide and Serious Crime Command are investigating.
At this stage, the deaths are being treated as unexplained and next of kin have been informed.
It is understood that officers are not looking for anyone else in connection with the deaths.
A spokeswoman for London Ambulance Service said: "We were called at 6.40pm to reports of an incident in Midcroft, Ruislip. We sent three ambulance crews, two single responders in a car, two officers and our hazardous area response team.
"Sadly three patients, a woman and two children, were dead at the scene."