The van window was smashed and the driver was threatened with a shotgun
A suspected burglar escaped when two men, one armed with a shotgun, held up a prison van carrying him to court.
The Serco van was forced to pull over by two men in balaclavas who smashed the driver's window and threatened him.
Prisoner Wayne Connor, 20, was driven away after the raid, a short distance from Feltham Young Offender Institution in west London.
No shots were fired and the driver has been treated for cuts and bruises. Two other prisoners in the van were unhurt.
Mr Connor was due to stand trial for assault, racially aggravated assault, burglary and aggravated burglary.
Police investigating the raid on Monday morning cordoned off several streets near Bedfont Road, which have since reopened.
Det Insp Alan Holford said Mr Connor and his accomplices are known to use a string of aliases and false documents.
"They used violence today and have access to weapons," Mr Holford said. "We also know that Mr Connor and the men with him use hotels so we are appealing to staff in hotels to keep an eye out for him."
The van was driving suspects from Feltham Young Offenders' Institution to Woolwich Crown Court, said a spokesman for Serco, the private firm contracted to provide prisoner transfers.
Another member of staff was unhurt during the incident.
The van was taking the inmate from Feltham Young Offenders Institute