Sexual abuse victims' groups have said it is "vitally important" police inform the public if sex offenders escape their watch, after it emerged the country's largest force has lost track of more than 100 registered criminals.
The Metropolitan Police admitted 123 sex offenders they are meant to be monitoring have failed to keep them informed of their details and their whereabouts remain unknown.
The figure has remained unchanged since 2008 with the majority of those at large being those lost four years ago, while some have been missing for up to 14 years.
Detectives believe that 48 of the sex offenders have managed to flee the country, despite warrants for their arrest being circulated nationwide.
Donald Findlater, director of research and development at the Lucy Faithfull Foundation, a UK-wide charity dedicated to reducing the risk of children being sexually abused, said: "Every registered sex offender going off the radar is a concern, here are 123 concerns.
"It's important that if within this 123 there are any sex offenders who pose a likely immediate risk to children, that information needs to be put across to the public.
"If the police don't know where they are then it is vitally important that police share that information with other agencies and the public."
He added that the sex offenders register has been modified many times since it was set up in 1997 and criminals now have to comply with strict regulations.
He said: "Many of them fail to keep police informed because they just don't want to be monitored, not because they're dangerous.
"The biggest risk to children across the country isn't posed by registered sex offenders, it's posed by people not on it at all."
He said police and other agencies are responsible for monitoring more than 4,000 convicted sex offenders in the capital.
A spokesman for the Met said: "The safety and protection of the public is paramount at all times when dealing with sex offenders.
"We take this matter extremely seriously and officers are proactively following lines of inquiry in order to trace these offenders to ensure that they are dealt with robustly for having breached the terms of their conditions."
In 2007 it emerged that police across the UK had lost track of 322 convicted sex offenders, with the Met having lost the most.
The sex offenders register contains the details of anyone convicted, cautioned or released from prison for committing sexual offences since 1997.
Those on it must keep in touch with police and update them if they change where they are living.