The prison service has launched an inquiry into how a prisoner with a history of violence escaped from Oxford's John Radcliffe Hospital.

Kenneth Wilson, 38, was back in custody on June 28 after handing himself into police in Humberside on the night of June 27.

He was picked up after calling police from a telephone box and arrested on suspicion of being unlawfully at large. He was taken to a nearby police station.

He was in the 12th year of an 18-year sentence for robbery when he was taken to the Oxford hospital for routine treatment on June 26.

He was not handcuffed and escaped at about 3pm when a prison guard accompanying him had gone to buy a cup of tea.

A prison service spokesman said a risk assessment had led staff to believe handcuffs would not be necessary.

She said: "Wilson was on a temporary release licence following a thorough risk assessment.

"A member of prison staff was there to assist with any problems, for example filling in forms."

She could not reveal details of the inquiry.

Wilson's return to custody brought to an end a major police hunt lasting more than 30 hours.

He was a category C (low risk) prisoner in the category B (medium security) Grendon Prison in Buckinghamshire.

Thames Valley Police warned he could be a danger to the public because of his violent history.

Officers followed up possible sightings, including one on the morning of June 27 in Cowley.

Wilson is Scottish but lived in Surrey before he was jailed.