It means the number of offenders who have gone missing despite supposedly being monitored by officers has more than doubled in two years.

Almost 700 of them have been on the run for more than a year, while police have not been able to trace some for more than a decade.

The disclosure raises serious questions about the effectiveness of the Sex Offenders' Register, which those convicted of sexual crimes are forced to sign.

Campaigners have urged the Government to overhaul the system, claiming it is "broken" and "out of control", leaving dangerous criminals free to abscond.

There are now more than 37,000 people on the register. Each is required to notify police if they change their address, while other are subjected to regular checks.

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However, figures suggest that on September 1 last year, police had lost track of 843 offenders, compared to 356 on the same day in 2009.

Of those, 690 had been untraceable for more than 12 months - up from 260 two years before.

There were 356 registered sex offenders at large on the same day in 2009, according to figures released following a Freedom of Information request to the National Policing Improvement Agency.

Lynn Costello, of Mothers Against Murder and Aggression, said: "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.

"But all the blame for this cannot be put at the doors of the police. How many paedophiles can be tracked down when the police are being asked to do so much paperwork?"

Malcolm Gilbert, operations director of Family Matters, Britain's largest counselling service for sex abuse victims, said the figures should be a "wake-up call" for the Government that the register is "broken" and needs urgent overhaul.

Last month the Metropolitan Police disclosed that the whereabouts of 123 registered sex offenders under its watch were unknown.

Some of them have been missing for up to 14 years, Scotland Yard said.

Detectives believe that 48 of them have managed to flee the country, despite warrants for their arrest being circulated nationwide.

The number of registered sex offenders in England and Wales has risen by nearly a quarter in five years - from 29,983 in 2006 to 37,225 last year, according to Ministry of Justice data.

Anyone given a jail sentence of 30 months or more for a sex crime is automatically placed on the register for life.

In the past six years, the number of offenders punished for breaching the conditions of the register has risen by more than 16 per cent to 1,492 last year.

From next year sex offenders put on the register for life will be able to appeal to have their name removed, following a human rights ruling.