Watch a preview of Prisons Unlocked. Panorama tracks down convictred drug smuggler Christian Ekkebus who walked out of an open prison.
Overcrowding in UK prisons is pushing unsuitable and dangerous criminals into low security open jails, according to prison and probation officers.
With the prison population at an all time high - over 90,000 inmates are currently locked up - the stark warning is that the public is being put at risk.
Colin Moses, of the Prison Officers Association has told a Panorama investigation: "The risk factor is massive".
It is unacceptable these institutions are coming under such pressure to take unsuitable prisoners that we see the risk of absconds and therefore a risk to the public
Shadow Justice Secretary Nick Herbert
His concerns are shared by the National Association of Probation Officers.
"I've certainly been worried for 15-18 months that individuals are being transferred into open jails who pose a risk to the public" said Harry Fletcher.
"The worry is they are not being properly assessed."
And that increases the risk of dangerous and violent criminals walking out of low security prisons and carrying out further serious crimes.
On the run
Panorama has discovered that over the last decade almost 14,000 prisoners left open jails without permission.
And they are not all inmates serving short sentences for minor offences.
The Government's own statistics reveal more than 130 murderers have gone on the run from the open estate in the last ten years.
More 'difficult' prisoners are being put into open jails
"It is unacceptable these institutions are coming under such pressure to take unsuitable prisoners that we see the risk of absconds and therefore a risk to the public," said Nick Herbert MP, the Shadow Justice Secretary.
Panorama investigated the case of a convicted murderer who walked out of an open prison in Scotland and killed again and a serial armed robber ,with a history of escape attempts, who was moved to the Open estate and promptly went on the run.
The programme also tracked down a Dutch drug trafficker serving a ten year sentence, who failed to return from weekend leave and is now back living in his native Holland.
The UK Justice Secretary Jack Straw - despite repeated requests - would not agree to an interview for the programme.
But in a statement he stressed public safety was paramount.
"All prisoners located in open conditions have been rigorously risk assessed" .
The Director of Scottish Prisons, Mike Duffy, admitted more 'difficult' prisoners were being put into open jails.
"We are planning and programming more for difficult prisoners to try and ensure they don't return to crime in the future," he said.
But he categorically denied there was a link between prison overcrowding and unsuitable prisoners being pushed into open conditions.
Those at the front line of the prison system disagree.
"We need to go back to rigorous risk assessment " said Colin Moses of the Prison Officers' Association.
"The other side of that is we put people in open conditions , they abscond, and continue their criminality. That puts everyone at risk ".
Panorama: Prisons Unlocked will be on BBC One at 8.30pm on Monday 5 May 2008.