The number of sex offenders has increased by almost 50 per cent in five years raising concerns that they cannot all be adequately monitored.

In some parts of the country the number of sex offenders the police have to keep track of has almost doubled, according to figures obtained by the Tories.

The Government will today publish its own figures on how many serious sex offenders or violent offenders are closely monitored under Multi Agency Public Protection Arrangements (Mappa).

The most recent figures obtained by the Tories showed that a total of 31,392 people were on the Sex Offenders' Register in 2007-08, a rise of 47 per cent from 2002-03 when 21,415 were registered.

Northamptonshire reported a 92 per cent increase over the period, from 200 to 383 sex offenders, while the number registered in London increased by 61 per cent from 2,085 to 3,351.

David Ruffley , the shadow policing minister, said: "The increase in the number of sex offenders being registered is a sign of progress but it also underlines the need to allow police to do their jobs.

"As a result of this worrying trend there will be pressure on police time to keep an eye on these offenders in local communities. That is why it is all the more important to cut the amount of police time wasted on unnecessary bureaucracy.''

Those on the register are required to notify the police if they change their name or address and some have to say if they plan to travel outside Britain, but the system has been criticised for being too lax.

Last year, a senior judge said it was "astonishing'' that homeless sex offenders were able to register a bus stop or park bench as their address.

Figures released last October also showed that the number of offenders who breach the terms of their release had risen by a quarter in two years, with one in 20 breaching their conditions.

The latest sex offender statistics will be published by the Government today alongside statistics relating to serious sex or violent offenders under Mappa supervision.

But the union representing probation staff has warned that changes in the way the Mappa statistics are presented will give a false impression that there has been a fall in the number of serious violent offenders.

Previous reports have provided data on the total number of people who go through Mappa in any one year but today's report is expected to give a snapshot from a single day, March 31.