Hundreds of high-risk prisoners, including one assessed as being likely to kill his original victim, have been released into the community as soon as they have served half their sentence, the probation union has warned.
Many offenders were recalled to custody within days of their release because of the clear risk they posed to victims, Napo said.
The union added that a change to the law six years ago means the Parole Board has no say over their release and prisoners are released automatically at the half-way point of their sentence, giving criminals no incentive for rehabilitation or reform.
Harry Fletcher, Napo's assistant general secretary, said: "It is scandalous that hundreds of prisoners are being released from custody automatically when they have completed half their sentence despite assessments that they are of high risk of harm to the public.
"There is evidence that this is putting the public at risk. The majority are recalled to custody because of threats to victims. There is clearly a need for the law to change."
Napo called for a discretionary system to be brought in for all prisoners serving two years or more so they would only be released if they no longer posed a significant risk.
It comes after the union's members found a series of cases where high-risk offenders were automatically being released.
One 40-year-old stalker, in the Thames Valley area, was released after serving half of his three-and-a-half year sentence, but was arrested within 48 hours after carrying out extensive internet searches and turning up at his victim's property. He is behind bars serving the rest of his sentence, but assessments suggest he is very high risk and is likely to kill his victim.
A Ministry of Justice spokesman said: "Reducing reoffending and protecting the public lies at the heart of our work. Offenders are rightly recalled to prison if there is a concern about their behaviour.
"Dangerous offenders with life and indeterminate public protection sentences will only be released once the Parole Board is satisfied their risk can be safely managed in the community."