Police were called at 7.29pm on Saturday, 27 August, to reports of a male stabbed on Uvedale Road in Dagenham.
Officers responded with the London Ambulance Service and London’s Air Ambulance.
A 15-year-old boy was located with a stab injury and treated at the scene.
He was taken to hospital where his condition continues to be treated as life-threatening.
Detectives from the Met’s East Area C.I.D. are investigating.
They would like to speak with anyone who witnessed the assault, or who saw three male suspects dressed in dark coloured clothing in the area.
There have been no arrests and enquiries into the circumstances are ongoing.
Witnesses and those with information that could help police should call 101 quoting reference CAD 6527/27 Aug.
Information can also be provided to the independent charity Crimestoppers, anonymously, by calling 0800 555 111.