Convicted murderer John Massey, who has escaped from Pentonville prison in north London, climbed the wall using a makeshift rope, the BBC understands.
It is thought Massey, 64, hid in the prison gym and went onto the roof before scaling the wall using netting.
Massey - serving life for a 1975 pub murder - was reported missing at 18:30 BST on Wednesday. Police say he should not be approached.
The Ministry of Justice has launched an investigation into the escape.
It is thought he used discarded netting, which may have been used for a sports activity, to fashion the makeshift rope.
Massey - one of the UK's longest-serving prisoners - was convicted of shooting dead a man with a sawn-off shotgun at The Cricketers in Clapton, Hackney.
BBC home affairs correspondent Danny Shaw said the escape was a major security breach causing deep embarrassment for the prison authorities.
It is highly unusual for inmates to escape from within the confines of a closed prison such as Pentonville, our correspondent added.
Police want anyone who spots Massey to call 999.
The BBC News website understands that Massey had previously escaped from Coldingley prison, in Surrey, in April 1994, and was subsequently reported to be living in Spain before being returned to prison.
Massey was released on parole in June 2007 - after spending the previous 18 months in an open prison on Derbyshire - and ordered to live in a bail hostel in Streatham, south London.
But he broke his curfew after a few months to go and live with his dying father and, after a number of days, was recalled to prison.
After two-and-half years, he was sent to Ford open jail, in west Sussex, from where he absconded.
He was arrested 10 months later and taken to Pentonville before Wednesday's escape.
Up to 1,250 category B and C male prisoners - not the most serious category A offenders - are housed at Pentonville.
Category B prisoners are defined as those "for whom the very highest conditions of security are not necessary but for whom escape must be made very difficult".
Category C prisoners are defined as those "who cannot be trusted in open conditions, but who do not have the resources and will to make a determined escape attempt.".
Conditions at Pentonville prison, which was built in 1842, have been heavily criticised by inspectors.
In March 2009, convicted arsonist Julien Chautard spent minutes inside Pentonville before he escaped by clinging to the underside of the security van he had arrived in .
He later handed himself in to police.