THE Prison Service says it has no plans to change its policy after a convict committed a string of armed robberies when he was allowed out of prison to attend a hospital appointment.

Politicians have demanded an inquiry after burglar Darren Williams was freed without supervision for an operation at Middlesbrough General Hospital.

The 28-year-old carried out a series of armed robberies after being allowed to leave Holme House prison, in Stockton for the day.

Williams was jailed for a further 12 years after pleading guilty to the crimes at Teesside Crown Court.

Stockton MP Frank Cook and former police superintendent Lord Brian McKenzie both said the incident should be reviewed.

Yesterday, a spokesman for the prison service said the incident was being investigated as a matter of routine. He added that Williams, from Stockton, had undergone a thorough risk assessment before he was allowed to go to hospital for an operation on his nose.

He said: "There is no plan to change our policy of temporary release for reasons like hospital appointments or on compassionate grounds once the prisoner has been properly risk assessed.

"Nonetheless, the policy is obviously kept under review and how it operates may be modified from time to time if there is anything to be learned.

"We are obviously concerned by this very serious case and an investigation is automatic.

"Many prisoners do leave for hospital visits and for compassionate reasons. The vast majority do not abscond or break the law."

Williams was serving a four-year and nine-month stretch when he was allowed to leave without a guard in March.

Teesside Crown Court heard he teamed up with Shaune Metcalfe and committed armed robberies at food stores and takeaways in Norton, near Stockton, nearby Stillington, and at Middleton St George, near Darlington, while armed with 7in blades.

Metcalfe, 24, also from Stockton, attacked a member of staff at the Norton food store and received a fourteen-and-a-half year sentence for unlawful wounding.

Williams, who has a record dating back 15 years including two for serious assault, admitted affray for threatening police with a knife when they tried to arrest him