Police are investigating the circumstances surrounding the death of a 3-year-old boy on the Lynton Estate in Bermondsey.
Officers were called at about 6.15pm on Monday to reports of a woman and a child injured at a flat on the Lynton Estate in Lynton Road, Bermondsey.
Three people were taken to hospital: a woman, aged 28, who was suffering a neck wound; a girl, aged 17 months, who was unhurt and a man aged 31 who was suffering serious stab wounds.
A 3-year-old boy found at the address was suffering stab injuries and was pronounced dead at the scene.
The Met says that it knows the identity of the dead child but not all next of kin have been informed.
The woman has now been discharged from hospital. The little girl did not require any treatment.
The man remains in a south London hospital. His condition is believed to be stable.
The death is being investigated by the child abuse investigation command major investigation team, based at Stratford led by DI Keith Braithwaite who can be contacted on 020 8217 6444.
"The news today of a little boy's violent death in Bermondsey is almost too horrific to imagine," says Simon Hughes MP.
"There will be huge shock and horror throughout the community, especially from all those of us who live very nearby.
"Thoughts and prayers of the country will be with the little girl also found in the flat, and all the family. Our thoughts are also very much with the police, NHS staff and other public servants who are dealing with this terrible tragedy.
"I have immediately sent wishes of support and solidarity to be passed on by Southwark borough's police commander."
Update 6.45pm: A 31-year-old man has been arrested. Officers have established that the boy was 3 years old, not 4 as previously reported.