Nearly 700 convicted sex offenders in the UK have been removed from a national register over the past four years, including 170 rapists and 157 child abusers.

Some 27 offenders who raped children under 16 years of age and three who committed incest with children under the age of 13 were among those who no longer have to register with police, the BBC reports.

A freedom of information (FoI) request made by the BBC has revealed that more than half of all applications made by offenders to be taken off the register since 2012 were approved by police.

However, request approval rates vary across the country. North Wales Police, for example, approved 90 percent of requests, while North Yorkshire Police granted just 7 percent of requests. Of the 44 police forces that can grant these requests, 40 responded to the BBC’s FoI request.

The removals follow a 2010 Supreme Court ruling that allowed sex offenders to appeal against being held on the register for life after two sex offenders argued that lifetime registration without a right to review was a violation of privacy rights.

But Peter Saunders, founder of the National Association for People Abused in Childhood, said that victims and survivors are “extremely concerned” about the removal of offenders from the register.

“Where somebody has become an adult offender, and has committed these kind of vile and serious crimes against children, then our view is that there isn’t really a case for removing them from a register even though the law says that that can happen.”

The Home Office has issued guidance saying that police must consider a number of factors, including representation from survivors and any evidence of risk of further sexual harm to the public before approving a request.

Adult offenders must wait 15 years after leaving prison to lodge an application, while juveniles can apply after eight years.

There were nearly 49,466 registered sex offenders in Britain as of March 31, 2015.