THE prison service was last night under pressure to order an inquiry into how a convict was able to carry out a string of armed robberies after being allowed out of jail without a guard for a hospital appointment.

Politicians demanded answers after hearing how burglar Darren Williams was freed for a routine operation at Middlesbrough General Hospital.

The 28-year-old was still doing time in Holme House Prison, Stockton, for a four year and nine month jail term.

Despite convictions stretching back more than 15 years, including two for serious assault, prison authorities decided he could be trusted to go to hospital alone.

Instead, he absconded and went on a vicious armed robbery spree with another criminal.

Yesterday, Williams from Stockton, was jailed for another 12 years after pleading guilty at Teesside Crown Court to two armed robberies at food stores in Norton, near Stockton, and Middleton St George, near Darlington.

He also pleaded guilty to an attempted robbery in Stillington and two burglaries all on March 16.

His accomplice, Shaune Metcalfe, 24, also from Stockton, received fourteen-and-a-half years.

Williams also admitted an affray when he threatened police with a knife as they closed in to arrest him. Metcalfe admitted unlawful wounding.

Teesside Crown Court heard how Williams went on the run five days after he was given unsupervised leave for his operation, said Ian Skelt, prosecuting.

Doctors were expecting to fix his broken nose. Instead, the prisoner never turned up.

Williams met up with Metcalfe and they armed themselves with seven-inch blades.

The pair burst into a supermarket in Norton and during the hold-up, Metcalfe slashed a member of staff's arm before they fled with only £42. The man needed 14 stitches.

Four hours later, the pair - who wore balaclava masks - attempted to rob a pizza shop at Stillington, near Stockton, but fled empty-handed.

Fifteen minutes later they held knives to the faces of terrified staff in the Middleton St George post office and escaped with £400.

When police gave chase the pair dumped their stolen Vauxhall Astra.

In a desperate bid to escape, Williams climbed on a house roof threatening officers with a kitchen knife.

The court heard how he had robbery convictions stretching back to when he was 13, when he hijacked other youngsters' bikes, and for assaults in 1992 and 1994.

He had been in jail since March 2 for burglaries and handling stolen goods.

Jamie Hill, defending, said that Williams had been extremely unsettled by being given hospital leave.

By the time of the robberies he was so much under the influence of Valium that he could hardly remember a thing.

Judge David Bryant warned Williams and Metcalfe that they could expect to serve ten years before parole.

Although prison authorities insisted Williams had been thoroughly vetted before being allowed out,the decision was greeted with amazement last night.

Cleveland Police Authority chairman Ken Walker said: "I only hope that lessons are learned as a result of this incident which resulted in a great deal of distress to people affected by this man's crime spree."

Stockton MP Frank Cook said: "I think there are questions to be asked and answers to be given.

"You can rest assured I will be asking the appropriate questions of the Prison Service."

Lord Brian McKenzie, a former Police Superintendent and advisory to the Home Office, said: "I am all for rehabilitation in the community but I am a firm believer that criminals should be tagged.

"It is clearly taking risks with the public when people are allowed to leave prison.

"If he simply required hospital treatment, I see no reason why a prison officer should not have stayed with him. I think the incident should be reviewed as soon as possible."

A Holme House spokeswoman said she could not discuss specific cases.

However, she added: "Before anyone is given a temporary release licence, they are thoroughly risk assessed."