The Home Office has been accused of "shocking incompetence" for leaving a legal loophole that prevents police from holding the DNA profiles of thousands of suspected sex offenders.
From October, police in England and Wales will be banned from indefinitely holding genetic information on people arrested on suspicion of sexual and violent crimes once they have been released without charge.
Police will be given the right to apply for data to be held for longer, the BBC reported.
But with this appeal process not yet in place, Labour said many forces are already following government directives to delete records ahead of the change of law.
Shadow home secretary Yvette Cooper said: "Theresa May's failure to prevent and deal with this incompetence shows she has not taken seriously enough the risks to rape convictions and crime from her policy."
The Home Office said: "Forces will be able to retain DNA from someone arrested and not charged for up to three years, but only with permission from the biometrics commissioner. And all DNA samples taken by police are checked against the national database before deletion."
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.
– National Association of Probation Officers (Napo) general secretary Harry Fletcher
More than 50 cases have been pulled together by the National Association of Probation Officers (Napo) after it approached members from across the country to provide examples of "complicated" medium risk sex 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.
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.
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.
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.
Speaking yesterday, Napo general secretary Harry Fletcher said: "The Government's plans are both chaotic and dangerous."
Serial sex offenders will be made to take lie detector tests on their release from jail, it has emerged. It follows a series of pilots launched by Labour in April 2009 that ran until October 2011 in the East and West Midlands.
It found offenders on lie detectors made twice as many admissions about contacting victims or entering an exclusion zone than without.
Offenders also reported that the tests helped them to better manage their own behaviour, according to the government.
Dangerous sex offenders face being forced to take lie detector tests on their release from jail, it has emerged. Paedophiles and rapists will be made to answer a series of questions about their intentions while hooked up to a polygraph machine.
The results could see the terms of their release restricted or, in the most serious cases, land them back in prison.
There are about 3,000 sex offenders being managed on licence in the community at any one time, with more than 750 considered to be the most serious - the category affected by the proposals.