Hundreds of prisoners, including murderers and paedophiles, who have been assessed as posing a serious threat to the public, are being transferred to open prisons. The Observer has obtained an extensive dossier of case studies involving more than 50 prisoners recently transferred to 'soft jails'.

The report compiled by probation officers at 10 open prisons across the UK includes the example of a Yorkshire man serving life for murder after driving his 4x4 car over his victim several times before hitting him with a wheel brace and then disposing of the body in a pond. Despite being classed high-risk, he was moved into an open prison and granted day release - at an address three miles from where the victim's family live.

They also reveal the case of a 42-year-old man held in an open prison in the south-west who is obsessed with his former partner. The man, who psychiatrists say has psychopathic tendencies, had attempted to murder his former partner's lover and was overheard in prison saying that he was going to 'get' his ex-partner. Despite this he was moved from a high-security jail.

Other examples of offenders moved to open prisons include that of a 25-year-old man convicted of blackmail and conspiracy to murder who was judged to be in the most dangerous category of offenders. Despite this, and warnings that he was likely to abscond, he was transferred to open conditions. Another case involved a man held for attempted murder with an axe, having attacked the victim on two previous occasions. Experts warned he still posed a high risk to the victim and had not attended any offending behaviour programmes.

The decision to transfer some of society's most violent members to open conditions is the result of a new Home Office strategy to ease prison overcrowding as the prison population neared 80,000.

'This is very, very dangerous for society,' said Brian Caton, general secretary of the Prison Officers' Association. 'The rising prison population means risk assessments are not being conducted properly and open prisons are not being used appropriately.'

Probation staff say open prisons have been given express orders by the Home Office's prison population management unit to accommodate an allocated number of high-risk prisoners each month. The news has dismayed public protection experts, coming at the end of a year which has seen a series of high-profile murder cases involving dangerous prisoners who were released early.

Over the past three months, prison staff report a huge rise in the number of dangerous prisoners transferred to the open estate, many against the express wishes of governors, who often were not consulted. Probation staff estimate that between a quarter and a third of prisoners transferred are, at any one time, still awaiting the correct risk classification.

'Watching this government's handling of the prison service is like watching a train crash,' said Edward Garnier, the Conservative spokesman on home affairs. 'The train left the station in 1997 and has been heading for the buffers. Along the way there were points where the government could have done things to make sure there wasn't a crash. Well, now there has been.'

The policy of moving more inmates into open jails is high-risk,' said Harry Fletcher, assistant general secretary of the National Association of Probation Officers. 'The government has a duty of care to staff and to safeguard the public. There should be an urgent review of this policy without delay.'

Of particular concern to caseworkers is the increase in the number of dangerous prisoners absconding. Examples include that of a man from North Wales with a history of serious violence and drug problems who was transferred, despite telling authorities he was not ready for a move, and who promptly absconded; a man jailed for trying to cut his partner's throat who absconded and whose victim had to be offered protection; and a 40-year old paedophile from Wales still on the run, having been transferred to open conditions in the summer. Since 2001, 4,300 prisoners have absconded from Britain's open prisons but the number has increased significantly in recent months, according to staff. Staff say violence within open prisons has increased significantly. Drug use, bullying and the use of smuggled mobile phones has also increased.

A Home Office spokesman denied claims that the public was now more at risk as a result of the re-categorisation strategy. 'Prisoners are stringently risk-assessed before being allocated to open prisons,' he said. 'Regular reviews of prisoners' security categorisation helps ensure they are held in the most appropriate conditions. The number of absconds in the last financial year was at its lowest level for 10 years in relation to the prison population. Absconds for 2006/07 to date remain in line with last year's figures, despite the population pressures, which indicates that the risk assessment continues to be applied rigorously.'