Some 2,300 sex offenders will be among the criminals whose supervision is outsourced to private contractors under Government reforms to probation, it was claimed on Saturday.
Around 3,200 gang members, 8,400 people convicted of domestic violence and 15,900 robbery cases are also among the "medium risk" offenders set for private supervision, the National Association of Probation Officers (Napo) said.
The public will be put at risk if such offenders are taken out of the care of the public sector and transferred to private firms such as G4S and Serco, the union said.
Napo general secretary Harry Fletcher said: "The Government's plans are both chaotic and dangerous."
More than 50 cases have been pulled together by Napo after it approached members from across the country to provide examples of "complicated" medium risk offenders.
Among the offenders who would be transferred to the private sector under Justice Secretary Chris Grayling's shake-up of rehabilitation, are a 32-year-old repeat offender in Greater Manchester who was convicted for violence against a child.
A 20-year-old woman from the West Midlands who has convictions for carrying knives, racial harassment, putting a man in hospital and attacking a police officer would also be transferred.
The dossier includes cases of child abuse, harassment by the internet, intent to cause grievous bodily harm, unlawful wounding and instances of repeated domestic violence. In many cases the offender is in a gang and the offending is gang related.
Medium risk is defined as "the offender has the potential to cause harm but is unlikely to do so unless there is change of circumstances".
Most medium risk offenders have multiple problems which can change dramatically leading to risk to victims, Napo said, while around 80% of further serious offences are committed by low or medium risk offenders.
Lower-risk offenders will be supervised by private firms and charities on a payment by results basis under Grayling's "rehabilitation revolution", while prisoners serving less than 12 months will undertake a period of rehab upon release for the first time.
Fletcher went on: "Splitting up offenders between the public and private sector according to risk threatens public protection."
"Offenders are generally not a compliant, problem free, group of people. They disproportionately suffer from mental illness, are four times more likely than the general population to misuse drugs and are 10 times more likely to have been in care."
"They need to be supervised by experienced staff who can motivate them and properly assess risk."
He added: "The Government plan to outsource the supervision of medium and low risk offenders is a disaster waiting to happen."
"The proposals are bureaucratic, costly and not thought through. They will undermine public protection and need to be independently evaluated before proceeding."
Napo added that the Probation Service has met or exceeded all its targets in the last year, while over the last six years reoffending rates for those on community orders have fallen by 5.6%.