Almost 90 prisoners who have gone on the run from Ford Open Prison in West Sussex are still unaccounted for, the BBC has learned.
Some of the prisoners who absconded have been missing since the 1970s.
Both Sussex MP Nick Gibb and the Prison Officers Association (POA) said mistakes were being made because of "constraints" in the prison service.
The Ministry of Justice said immediate and major changes had been ordered to tighten up the system.
Ford and other open prisons have seen a series of high-profile disappearances in recent weeks.
Sussex Police said 89 inmates who had gone missing from Ford were unaccounted for.
Thirty-four of those prisoners have been missing since 2004 but others absconded decades ago.
The prison can take up to 557 inmates and accepts offenders with under two years left to serve.
'Quick steps taken'
In November, the force found there were a number of people who had absconded whose cases had not been regularly reviewed, and a dedicated team was set up to review each case.
Supt Lawrence Hobbs said there had been 90 inmates at large, but armed robber Simon Rhodes-Butler was arrested on Thursday.
"Of the 89 outstanding, only eight have convictions for violence and we are prioritising our efforts to trace and arrest them," he said.
"I want to reassure people that we recognised some time ago that our processes needed improving.
"We have taken some really positive action and have a team working on this issue."
This week, the force was criticised for waiting four years to tell the public there was a convicted murderer on the loose.
Supt Hobbs said police did not routinely make media appeals to the public when people went missing, but considered each case individually, taking into account the risk.
He said the vast majority were serving jail terms for offences such as fraud, dishonesty and theft, and he added: "We are absolutely confident the risk to the public remains low."
'Public are at risk'
But Mr Gibb, Conservative MP for Bognor Regis and Littlehampton, said: "It's becoming a pattern. It says to me the wrong people are being sent to Ford Open Prison."
He said placing a prisoner in open conditions required a very rigorous risk assessment, but he was not convinced the process was rigorous enough.
"I sense that because of the constraints on capacity in the prison service that mistakes are being made in that assessment process," he added.
Steve Gillan, general secretary of the POA, said open prisons provided necessary rehabilitation but officers were noticing "time and time again" the wrong sort of prisoner being put in open conditions, putting the general public put at risk.
"There aren't enough prison officers because of budget cuts, we're too overcrowded, mistakes are being made, and I think people are being rushed through the system incorrectly," he said.
He said he believed the justice secretary was considering a tagging system, but that the technology was not yet available.
A spokesman for the Ministry of Justice said: "The justice secretary has been clear that keeping the public safe is our priority and has ordered immediate and major changes to tighten up temporary release processes and open prison eligibility."
He said absconds had reached record lows under this government and were down 80% over the past 10 years, but each and every incident was taken seriously, with police contacted as a matter of urgency.
Justice Minister Jeremy Wright said open prisons played an important role in preparing prisoners for release in the last few months of their sentence.
"If we did not have them, all the expert advice is that reoffending would be higher", he added.
The issue of open prison absconders hit the headlines after Michael Wheatley, nicknamed the "Skull Cracker", absconded from HMP Standford Hill in Kent on 3 May.