Almost 700 convicted sex offenders have been removed from the register in the past four years, the BBC has learned.
Since 2012 at least 170 rapists and 157 child abusers were told they no longer had to register with the police.
They include people convicted of raping boys and girls, incest, and taking indecent images of children.
More than 50% of applications - allowed under a 2010 Supreme Court ruling - were successful, a Freedom of Information request revealed.
Figures showed North Wales Police approved 90% of requests, while neighbouring Dyfed-Powys Police did not approve any.
There were 49,466 registered sex offenders in the UK as of 31 March 2015 and they are only able to appeal 15 years after leaving prison.
The Home Office has issued guidance stating offenders should only be removed if it is no longer necessary for them to be registered to protect the public from sexual harm.
Among those removed from the register include 27 offenders who raped people under 16 years old and three who committed incest with children under 13.
Analysis, by Danny Shaw, BBC home affairs correspondent
Theresa May brought in these new rules through gritted teeth after the Supreme Court declared that, with no right of review, requiring sex offenders to register their address with police and inform them of travel plans was disproportionate and incompatible with the right to privacy.
In 2012, when the rules came into force, the Home Office said police had to conduct a "robust review" and be "satisfied that it is not necessary, for the purpose of protecting the public from the risk of sexual harm", for a sex offender to continue to register.
This comprehensive set of figures appears to demonstrate that police are interpreting that guidance very differently.
How else can you explain the huge variations between North Wales, where 90% of requests not to register with police succeeded and Dyfed-Powys where none did; or Northumbria, 79.2% and North Yorkshire, 7.1%?
David Hines, founder of the National Victims' Association, said it was "outrageous" that people who had committed "horrific crimes" were no longer on the register.
He said: "I think the public will be outraged as well. These people should stay on the register. This is not protecting the public."
A Home Office spokesman said: "Public safety is at the heart of all decisions taken by police.
"Those who continue to pose a risk will remain on the sex offenders register - if necessary, for life."
A spokesman for North Wales Police said removals from the register are only authorised after a senior officer has reviewed the case.
How did the change come about?
The Sexual Offences Act 2003 made all sex offenders sentenced to more than 30 months in prison subject to indefinite notification requirements, without an opportunity for review.
But in 2010 the Supreme Court ruled that under human rights laws offenders in England and Wales should have the opportunity to prove they had reformed.
The offenders who brought the challenge said permanent inclusion on the register with no chance of a review was disproportionate.
And although the Supreme Court said an appeal should be possible, it underlined that it was lawful to monitor someone for life if they are a danger to society.
At the time of the ruling Home Secretary Theresa May said: "We will deliberately set the bar for those appeals as high as possible. Public protection must come first."
A total of 40 UK police forces responded to a Freedom of Information request from BBC News, out of 44 forces that can approve applications.
They reported 1,289 requests to be removed had been received, out of which 679 (53.7%) were approved.
Information about the offences people had committed was also provided by 35 of the forces.