Police were called by the London Fire Brigade shortly after 7pm on Thursday, 16 December to reports of a fire at a residential address on Collingwood Road in Sutton.
Officers attended along with the London Ambulance Service.
The fire was extinguished and four children – two 3-year-old boys and two 4-year-old boys – were recovered from the address. The boys are all related.
They were treated at the scene before being taken to two south London hospitals. Very sadly, all four were pronounced dead.
Next of kin are aware and will be provided with specialist support.
Road closures are in place and emergency services remain at the scene.
A 27-year-old woman has been arrested on suspicion of child neglect and remains in police custody.
Superintendent Rob Shepherd said: “The thoughts of everyone at the South Area Command Unit are with the family and friends of these four young children, who have suffered such a devastating and tragic loss tonight. I know these sentiments will be shared by the local community, Sutton and London as a whole.
“We remain on scene with our emergency service colleagues and will be working through the night and beyond. At this very early stage the cause of the fire is unknown and will be investigated.”