An investigation is underway after an “extremely dangerous” sex offender walked out of an open prison at the weekend and is still on the loose.

Police are hunting for Paul Robson, 56, who was discovered missing from HMP North Sea Camp, an open category D prison in Boston, Lincolnshire, after the morning roll call at 6.30am.

Police warned that Robson, jailed for life in August 2000 could be “anywhere in the country,” having had at least eight hours to travel before his escape was discovered.

Category D open prisons have minimal security as prisoners are prepared for integration back into normal life by working in the community on licence. “You can just walk out,” said a prison service source.

Ministry of Justice (MoJ) sources said his case would now be investigated as to whether he had been correctly assessed for transfer to an open prison. He faces being returned to a closed higher security prison if and when is caught.

He is believed to be at least the fourth prisoner from HMP North Sea Camp to walk out of the prison in the past decade. One inmate Shied Riasat, who was serving an indefinite sentence for arson, spent nearly nine years on the run before being apprehended last year.