Nearly 850 registered sex offenders are at large, with nearly 700 of them missing for more than a year, it has been reported.
One has been off the radar for nine years, the Daily Mail said.
Anyone jailed for at least 30 months for a sex crime is placed on the Sex Offenders' Register for life and required to inform police if they change address.
According to figures obtained by the newspaper, following a Freedom of Information request to the National Policing Improvement Agency, a total of 843 people on the register were at large on September 1 last year.
Some 690 of those had been untraceable for more than 12 months, it was reported.
The figures show the number of registered sex offenders at large has more than doubled in two years, with just 356 missing on September 1 2009.
Lynn Costello, of Mothers Against Murder and Aggression, told the newspaper: "The figures don't surprise me because nobody has really had a control of the Sex Offenders' Register since it was brought in.
"These people are offenders who want to move around and remain underground and undetected."
She added: "But how many paedophiles can be tracked down when police are asked to do so much paperwork?"
A Home Office spokesman said: "The public deserves to be protected from predatory sex offenders.
"We have consulted on a range of measures to tighten the law and close loopholes. These include making sex offenders notify the police of all foreign travel, and ensuring they cannot seek to avoid being on the Sex Offenders' Register if they change their name.
"We will be making an announcement shortly."