Police are warning the public not to approach 64-year-old John Massey, who was serving a life sentence for the murder of a man at a pub in east London in 1975.
Authorities have not revealed how Massey escaped, but the BBC reported that he scaled a wall using a makeshift rope.
Massey has eluded custody before. Britain's media reported that after being granted parole, he broke the terms to visit his terminally ill father in 2007. He escaped again in 2010 after hearing news that his sister was gravely ill.
The prison, Pentonville, houses up to 1,250 male prisoners. It is more than 150 years old and was considered a model for prison architecture.
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