The prison service says it has no plans to change its policy after a convict was allowed to leave jail without a guard only to commit armed robberies.

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

The 28-year-old committed a string of armed robberies after being allowed to leave Holme House prison in Stockton for the day.

Politicians demanded answers after the Williams was jailed for another 12 years after pleading guilty to the crimes at Teesside Crown Court. Both Stockton MP Frank Cook and former police superintendent Lord Brian McKenzie said the incident should be reviewed.

Today 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 his nose to be fixed.

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 years and nine month stretch when he was allowed to leave without a guard in March. Teesside Crown Court was told that he teamed up Shaune Metcalfe and committed armed robberies at food stores and takeaways in Norton, near Stockton, Stillington, near Stockton and Middleton St George, near Darlington. The pair were armed with seven inch blades.

Metcalfe, 24, also from Stockton, attacked a member of staff at the Norton food store. He received a fourteen-and-a-half year sentence for unlawful wounding. Williams has a record dating back 15 years including two for serious assault. He admitted affray for threatening police with a knife when they tried to arrest him.