Police have lost track of almost 400 registered sex offenders they should be monitoring, include a child rapist who vanished two years ago, official figures have revealed.
Others have been off the police radar for much longer, one for 14 years.
Claude Knights, of the child protection charity Kidscape, said: "The horrifying prospect is that these offenders will have rebuilt their lives in communities who don't know them.
"Re-offending is high and particularly among those who have left the support system that was designed to help them."
Everyone on the register – currently around 60,000 – is required to say where they are living. They are monitored by police, probation officers and the prison service under the multi-agency public protection arrangements (MAPPA).
But a total of 396 registered sex offenders have vanished after moving home without telling the authorities their new address.
Some of those missing are foreign nationals who may have returned to their own countries. That could include failed asylum seeker Patrick Kanda, 36, who was convicted of child rape.
He was monitored for several years after he was released from prison, but vanished two years ago from his home in Dagenham, Essex.
He is one of 167 offenders who have stopped reporting to monitors at the Metropolitan Police.
West Midlands Police has "lost" 39 registered offenders, Greater Manchester Police is missing 25, and Essex and Sussex have no contact with 11 offenders.
There are offenders missing from their homes in most of the UK's 45 forces, but only Scotland says it knows where all its 4,775 sex offenders are living.
The figures were revealed in response to a Freedom of Information request by the Press Association news agency.
Jim Gamble, who runs child protection consultancy Ineqe, said: "Offenders who have chosen to go off the radar should be pursued and punished.
"By disappearing they are removing the incentive for staying in the system for the many others who choose to comply."
A Home Office spokesperson said: "The UK has some of the toughest powers in the world to deal with sex offenders and we are committed to ensuring the system is as robust as possible.
"Under this government more adult sex offenders are being imprisoned, and they are being imprisoned for longer. The latest published figures show there are more than 11,000 sentenced adult sex offenders in prison – up by more than 2,500 since 2010.
"We have also reformed civil orders by introducing new measures that give police and the courts further powers to restrict and monitor the activities of sex offenders and those who pose a risk.
"It is for the police to manage offenders in their area, but we work closely with forces to ensure legislation is effective and that officers have all the tools they need."ur days.