At approximately 0725hrs on Wednesday, 31 July police in Wandsworth were called to Garratt Lane, SW17 following reports of a gas explosion.
Officers, London Ambulance Service and London Fire Brigade attended and found a wall to a block of flats damaged following a suspected gas explosion.
Emergency crews worked to assist those injured as a result of the damage caused.
One man aged in his eighties has been taken to hospital by London Ambulance Service. He remains in hospital in a critical condition.
A man from a neighbouring property was also taken to hospital for treatment for shock.
Three other people were treated at the scene for minor injuries. There are no other reported injuries.
Work at the scene is ongoing. No arrests have been made. A police cordon has been put in place. Motorists are advised to avoid the area.
Emergency crews continue to deal at the scene.
Sergeant Jon Nicolaou from South West BCU said: "Sadly an elderly man remains critically injured following this gas explosion. Our thoughts are with him. Another man from a neighbouring property has been taken to hospital as a precaution. Three others were treated at the scene for minor injuries.
“The Health and Safety Executive will be informed in due course and the Gas Board are at the scene now. They will be investigating the cause of the gas explosion which is believed to have taken place on the ground floor. Our enquiries continue. All emergency services continue to work together at the scene along with the local authority."