The Independent Office for Police Conduct has launched an independent investigation after officers attended an address in Camberwell.
Police were called to a residential property on Harleyford Road, SE11 at 03:30hrs on Tuesday, 25 June following reports of an attempted break-in.
On arrival, officers were met by a white man in his 50s armed with a knife. It is understood the man may have been suffering from poor mental health.
It was also established that another person was inside the address at risk.
Officers forced entry to the address and the man sustained a serious injury which is understood to have been self-inflicted. It is also understood a Taser was used by officers.
The man was taken to hospital by the London Ambulance Service where he remains in a stable but critical condition.
The IOPC has declared the matter to be subject to an independent investigation.