A prolific robber who absconded from an open prison should not have been moved there if he posed a danger to the public, union bosses have warned.
John Rooney, 47, who is half-way through a 24-year jail sentence for kidnap and more than 20 robberies, walked out of HMP Springhill in Aylesbury on Monday.
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".
Greater Manchester Police are also involved in the investigation, with Rooney linked to the Manchester area, including family who live in Oldham.
Rooney was jailed in 2003 for 24 years for a string of offences, 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 was sentenced to 20 years in prison at York Crown Court for offences including robbery, burglary and use of a firearm in May 2003.
Just months later he was handed a further four years at Manchester Crown Court for robbery, kidnap and false imprisonment committed in the Manchester area, while a 20 further offences of robbery and burglary were also taken into consideration.
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.
Chief Inspector Jim Troisi from Oldham police station said: "Greater Manchester Police are supporting an investigation to find Rooney and we are working closely with our colleagues in Thames Valley CID and the Force Intelligence Bureau so that we can locate him as soon as possible.
"Whilst there is no intelligence at present to suggest that Rooney is in the Greater Manchester area, it is possible he may come back to the region due to links to Manchester, including family links in Oldham.
"If any members of the public see Rooney, then I would ask them please do not approach him directly, but call 999 immediately.
"And to anyone that may be reading this that knows Rooney and has any information on his current whereabouts, please call police on 101."
Rooney has tattoos on both his hands and fingers as well as up his right arm, and possibly other tattoos on his face.
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.