A violent criminal who absconded from an open prison should never have been there if he posed a danger to the public, union bosses have warned.
John Rooney, 47, who is less than half-way through a 27-year jail sentence for kidnap and robbery, walked out of HMP Springhill in Aylesbury yesterday.
Thames Valley Police have warned the public not to approach Rooney because there are "substantial grounds for believing Rooney represents a risk of harm to the public".
He was jailed in 2003, but had been moved to the open prison where longer term prisoners are prepared for release.
Glyn Travis, a spokesman for the Prison Officers Association (POA), said the union has repeatedly raised concerns with the Ministry of Justice about dangerous inmates being moved to open prisons.
He said: "Anyone who's in an open prison shouldn't be considered a risk to the public, they shouldn't be there if they are.
"We believe that prisoners who still pose a risk to the public are being sent to the open prison estate too early and therefore when they abscond the police immediately alert the public that this person is a danger.
"Our concern is not the number of prisoners absconding, this is going down, but the type of prisoner absconding."
He added that open prisons play a fundamental role in the justice system, but that prisoners must have first served significant time in closed prisons and demonstrated an appropriate level of rehabilitation before being transferred to one.
Rooney left HMP Springhill in Aylesbury to take a lift to Bicester at around 6.30am on Monday and was last seen on a bus travelling towards Oxford just half an hour later.
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.
A Prison Service spokesman said: "Public safety is our top priority and we have made major changes to tighten up temporary release processes.
"Absconds are down 75% over the last 10 years, but each and every incident is taken seriously and the police are contacted urgently.
"Open prisons and temporary releases are important tools in rehabilitating offenders, but not at the expense of public safety."
In 2013/2014, a total of 137 inmates absconded from open prisons in England and Wales, which have recently been the subject of controversy after a spate of criminals walked out.
The most high-profile of those was Michael Wheatley - known as the "Skullcracker" - who sparked a nationwide manhunt after absconding from HMP Standford Hill on the Isle of Sheppey in Kent in May last year.