A prisoner serving a 27-year jail term for kidnap and robbery has absconded from an open jail, Thames Valley Police said.
The public has been warned not to approach John Rooney, 47, because there are "substantial grounds" for believing he represents a "risk of harm to the public".
Rooney left HMP Springhill in Aylesbury to take a lift to Bicester at around 6.30am on Monday. He was last seen on a bus travelling towards Oxford just half an hour later.
He was handed a 27-year jail term for the violent offences of kidnap and robbery in 2003, but had been moved to the open prison where longer term prisoners are prepared for release.
Detective Inspector Joe Banfield, senior investigating officer, said: "Thames Valley Police is working closely with the Prison Service to try and locate Rooney.
"Members of the public are advised not to approach him, but to call 999 immediately."
He added that anyone with information about his whereabouts should call police on 101 or Crimestoppers anonymously.
Rooney is described as white, 5ft 8ins and of slim build, with brown hair and has known links to Manchester.
He has tattoos on his left arm, his left wrist, hand and fingers, as well as on his right hand and letters on his right fingers, and a scar on his right hand.
He may also have other tattoos, particularly on his face, but these may not be immediately visible.