The Prison Service has launched an internal inquiry into how a convicted gunman was freed by two masked men.
It is the third time Farnan has managed to escape
Joe Farnan, 27, arrived at Hammersmith Hospital by ambulance on Monday afternoon when two men - one armed with a gun - opened the rear doors.
The masked men threatened three prison officers and forced them to uncuff Farnan before all three fled by car.
Farnan, from Bermondsey, south London, was in prison at Wormwood Scrubs for conspiracy to rob, and other crimes.
He was being taken to hospital after complaining of fits while in prison.
Brian Caton, general secretary of the Prison Officers' Association, said Farnan's escape highlighted failings in the prisoner categorisation system.
Increasing prisoner security might mean a "huge amount of money being spent" but it would keep the general public and prison officers safe, he added.
But a Home Office spokeswoman said: "This prisoner was not submitted for category A status as he did not fulfil the relevant criteria.
"Only high risk category A's on planned moves, where there is an intelligence suggesting armed resistance, would be likely to get police armed cover. None of this applied in this case."
The spokeswoman also said that escapes from prisons were the lowest since records began with three last year in comparison to 1992 when there were over 200.
It is the third time Farnan has managed to escape from custody.
The gang's escape vehicle, a silver Volvo, was found abandoned in the early hours of Tuesday in Ealing, west London.
In May 2000, following an appearance at Tower Bridge Magistrates' Court where he was appearing charged with armed robbery, Farnan absconded from a prison van transporting him along Tower Bridge Road.
Police recaptured him about three months later at Surrey Quays shopping centre.
In April 2001, Farnan was sentenced to 16 years in jail at the Old Bailey for robbery, conspiracy to rob, possession of an imitation firearm and escaping from lawful custody.
He later escaped from Standford jail in Kent, before being given an indeterminate sentence at Harrow Crown Court last July for conspiracy to rob, aggravated vehicle taking and escaping from lawful custody.
Police described Farnan as white, of medium build with dark brown hair and blue eyes.
He is about 5ft 10in and has a distinctive scar on his forearm.
The prison officers threatened on Monday were unhurt, but are being treated for shock. No shots were fired.