A 16-year-old boy is in a critical condition after a collision between a police car and a moped being ridden by three teenagers in south-west London.
The crash happened at 02:15 BST on Sunday in South Park Road, Wimbledon.
All three boys were taken to a south London hospital for treatment.
The moped was believed to have been involved in an attempted robbery and was being monitored by the National Police Air Service helicopter, said the Metropolitan Police.
A second 16-year-old suffered a serious injury to his leg and a 15-year-old sustained minor injuries.
All three were arrested at the scene and two large knives were recovered.
The moped had been reported to police as lost or stolen, on 12 July.
The Met said the Directorate of Professionals Standards has been informed and the incident had been referred to the police watchdog.
In a statement, the Met said the moped was not being “pursued by police vehicles on the ground” at the time of the collision but “was monitored by police helicopter”.
“The moped was in collision with the rear offside of a marked police car, which was being driven to a position ahead of the moped,” it said.