Staff believe Andrew Crisp may have had outside help to scale the prison walls.

The 36-year-old was sentenced to eight-and-a-half years for rape and arson at Nottingham Crown Court in April 1997.

Police in Lincolnshire and Nottinghamshire are searching for the man, and prison governor Barry McCourt has announced an unrgent investigation into the escape.

Security cleared

A Prison Service spokesman said Crisp's security classification was downgraded after good behaviour to allow him to live in accommodation separate from the main prison, but within the jail's boundaries.

The jail was recently inspected by HM Chief Inspector of Prisons who did not raise any concerns about the accommodation provided or the level of security.

But a Prison Service spokesman confirmed Crisp would be returned to more secure accommodation once he has been recaptured.