A legal challenge to government plans to privatise some probation services in England and Wales has been launched by the probation officers' union Napo.
It says the government's decision to split up services has put probation staff and the public at risk.
It comes after the government announced a list of preferred bidders to buy and run private companies to supervise low and medium risk offenders.
Justice Secretary Chris Grayling said changes will help reduce reoffending.
"These reforms are all about changing lives. We cannot go on with a situation where thousands of prisoners are released on to the streets every year with no guidance or support, and are simply left to reoffend," Mr Grayling said.
"These reforms will transform the way in which we tackle reoffending."
Under the changes, the probation service - which was split in two earlier this year in preparation for the new system - will continue to supervise high risk ex-offenders.
The National Probation Service (NPS) will supervise and rehabilitate 31,000 high-risk offenders.
New Community Rehabilitation Companies (CRC) will supervise 200,000 low and medium risk offenders, including 45,000 short-sentence prisoners who currently do not receive any probation monitoring.
The contracts are worth around £450m a year over seven years.
Clive Coleman, BBC Legal Correspondent
Reoffending is the nut that no criminal justice system has ever fully cracked. Rates here remain stubbornly high.
Of the 45,000 offenders released from prison every year having served a sentence of a year or less, nearly 60% reoffend within a year of release. These offenders have not previously been supervised by probation.
So the government is trying something very new and ambitious by opening up the market to a range of rehabilitation providers from the private, voluntary and social sectors.
Critics say this is untried, a leap into the unknown that could put the public at risk. They point out that the 35 probation trusts in England and Wales had all been graded good or excellent by Her Majesty's Inspectorate of Probation. And, if it ain't broke….
Napo's point is that the service is being fractured for privatisation and communication and information sharing which is critical to assessing risk, will be much more challenging.
The government says its reforms are about bringing together the best of the public, private and voluntary sectors to battle reoffending, and that something new has to be tried.
On Wednesday, ministers named the private firms they expect to take on the running of the 21 CRCs.
However, Napo says the new way of working has already caused problems.
It says protection laws stopping private companies accessing personal data mean probation officers working for CRCs have found themselves unable to look at an offender's full criminal history.
Napo says it means staff cannot assess individuals safely or make a rational judgement as to their risk to the community.
The union now wants courts to decide whether the government's decision to privatise part of the service was reasonable.
Ian Lawrence, general secretary of Napo, said the Ministry of Justice had "refused to listen to our concerns".
In a letter to the MoJ - which has been seen by the BBC - the union's lawyers, Slater & Gordon, say a number of "real and immediate" risks have been posed to probation staff, as well as to the public.
It calls for the government to make public results of safety tests to ensure CRCs are capable of running the service.
The letter also referenced examples of when members of staff have been put at risk because of the information access problem, and their subsequent lack of knowledge about the background of an offender.
- An occasion when a female probation officer was subjected to inappropriate sexual behaviour from a male offender because she was unaware of his history. She had to be signed off with stress, the letter said
- An inability by staff to accurately make a judgement call on the suicidal tendencies of an offender, again, because of a lack of information
One probation officer, who spoke to BBC News anonymously, said they had been managing an offender "blind".
"I cannot access his case files. He has got a history of previous sexual, violent offences, but I have no way of getting the detail."
"I'm currently looking for new employment. This is not the job I want to do anymore, we are not protecting the public," the officer added.
However, Mr Grayling said the reforms would bring together "the best of the public, private and voluntary sectors to battle against reoffending".
"I am really pleased that we will be deploying the skills of some of Britain's best rehabilitation charities to help these offenders turn their lives around," he added.