Ryan Byrne, 34, was mistakenly freed from the prison in south London, Scotland Yard said. He was sentenced to a total of nine years’ imprisonment earlier this year for burglary, robbery and theft.
The Prison Service said it was investigating the incident.
Byrne is described as white, 5ft 11in and of medium build. He has blue eyes and a goatee beard.
A Yard spokesman said police were informed by the Prison Service on Friday that Byrne had been released in error.
A Prison Service spokeswoman said: “Releases in error are very rare but regrettable occurrences. The number of incidents have fallen significantly in recent years with figures down by a quarter compared to 2009/10 but every incident is taken extremely seriously and we are not complacent.
“We are investigating this incident. The recapture of this prisoner is now a matter for the police.”
The Sun newspaper said Byrne had been taunting police by posing for pictures on the run. He was shown grinning in front of a police van, possibly in London’s Chinatown, and enjoying a pint in a pub.
A source told the newspaper: “Byrne was sitting in his cell having a cigarette when he was told he was free to leave. He couldn’t believe it. But he didn’t protest and has been enjoying his freedom.”
Police have checked his mother’s house in Acton, west London, but he is still on the run, it added.
Anyone who has seen Byrne or has any information on his whereabouts is asked to contact police on 101.