A convicted murderer has escaped from Pentonville prison, in north London, Scotland Yard says.
John Massey fled the Islington jail at about 1830 BST. He is considered potentially dangerous and should not be approached by the public, police say.
Massey, 64, is serving life imprisonment for shooting dead a man at an East London pub in September 1975.
The Ministry of Justice said it had launched an investigation into his escape.
It is understood Massey was not in transit at the time and escaped from within the prison.
BBC home affairs correspondent Danny Shaw says the escape is deeply embarrassing for the prison authorities and will inevitably raise questions about prison security.
It is highly unusual for inmates to escape from within the confines of a closed prison such as Pentonville, our correspondent adds.
There have only been three such escapes in the past three years in England and Wales.
Massey was serving life after being convicted of shooting dead a man with a sawn-off shotgun in Hackney.
In 2007, he escaped while on day release while being transferred from a high-security jail to an open prison.
Police want anyone who spots Massey to call 999.
Conditions at Pentonville prison, which was built in 1842 and holds 1,200 male inmates, have been heavily criticised by inspectors.
How the fuck does someone escape from within the confines of a prison?!