Police were called by London Ambulance Service at 4.08pm on Monday, 14 December to an address in Old Meadow Lane, Hounslow.
Officers attended with London Ambulance Service paramedics. A 25-year-old woman and a two-year-old girl were pronounced dead at the scene.
The woman and the girl were related to each other. Next of kin have been informed and are being supported by specialist officers.
An investigation is underway led by detectives from the Met’s Specialist Crime Command (Homicide).
While the investigation is still in its early stages, officers do not currently believe anyone else was involved in this incident.