Two children have died and a baby remains in a critical condition after what police believe was a knife attack in Carshalton.
Officers were called about 10.30pm last night to what they say was a domestic incident in Park Lane.
A five-year-old boy, a girl, four, and a six-month-old girl were rushed to hospital with suspected stab wounds.
The older boy and girl later died.
A 39-year-old man and a 35-year-old woman were arrested at the scene and have been taken to separate south London police stations for questioning.
Postmortem examinations are being arranged and murder squad detectives and the Child Abuse Investigation Command are investigating.
Police said they believed they knew the identity of the dead children are awaiting formal identification.
Neighbours said the five were a Tamil family from Sri Lanka who moved in to the two-storey detached house less than a year ago.
The injured boy and girl died within minutes of each other just before midnight after being taken to hospital.
The injured baby girl's condition has improved from critical to serious this morning.
One neighbour saw paramedics carrying the children to ambulances last night.