Police have appealed for help in tracing a "potentially dangerous" murderer who went on the run after escaping from Pentonville Prison.

John Massey, 64, made off from the jail in Islington, north London at around 6.30pm on Wednesday, Scotland Yard said. The convicted killer was sentenced to life imprisonment for the murder of a man at a pub in Hackney, East London in 1975.

It has not yet been revealed how Massey escaped from Pentonville, but an investigation into how he managed to elude staff is due to be launched by the the Prison Service.

A Prison Service spokesperson said: "At approximately 6.30pm on Wednesday 27 June a prisoner was found to be missing from HMP Pentonville. Police were immediately informed and the matter will be the subject of a thorough investigation."

Scotland Yard have advised members of the public not to approach the "potentially dangerous" offender and instead call 999 if he is spotted.

A police spokesman said: "Police are appealing for help in tracing a convicted murderer who has escaped from HMP Pentonville. He is considered potentially dangerous and should not be approached by the public if he is seen."

Pentonville houses up to 1,250 category B and C male prisoners, rather than the most serious category A offenders. According to reports, Massey was released on parole in June 2007 after spending the last 18 months of his life sentence in an open prison in Derbyshire preparing for freedom.

Under his parole terms he was ordered to live in a bail hostel in Streatham, south London, under a curfew. After several months he broke his curfew and spent a number of days living with his dying father, according to the Guardian.

He was immediately recalled to prison but decategorised after two-and-a-half years before being sent to Ford open jail in West Sussex. Massey reportedly walked out of the prison after hearing news that his sister was gravely ill. He was rearrested 10 months later and taken to Pentonville before his latest escape.

Anyone with information of Massey's whereabouts is asked to call Islington Police on 020 7421 0296, or people can inform Crimestoppers anonymously on 0800 555 111.