The number of paedophiles, rapists and sex attackers rated a danger to the public has increased by half in only five years, it emerged last night.

The big rise in the number of convicts placed on the Sex Offenders Register - which now stands at 31,392 - came amid allegations that ministers are concealing the extent of their reoffending.

Some rapes and killings by supervised criminals are being excluded from official figures.

The sex offenders' register figures, obtained by Tory police spokesman David Ruffley , show a 47 per cent increase in England and Wales - from 21,415 in 2002/03 to 31,392 in 2007/08.

'As a result of this worrying trend there will be pressure on police time to keep an eye on these offenders,' Mr Ruffley said.

'That is why it is all the more important to cut the amount of police time wasted on unnecessary bureaucracy.' Every single police force area has seen an increase in the number of registered sex offenders.

Especially big rises were seen in London where the numbers leapt by 61 per cent from 2,085 to 3,351. In Northamptonshire the numbers were up by 92 per cent from 200 to 383.

Meanwhile, a BBC Panorama investigation into Mappa, the multi-agency public protection arrangements for released sex and violent offenders, found that only those placed in the top two risk levels - two and three - are tracked in detail by police and probation services and their reoffending rates made public.

The majority are placed in the lowest tier - one - and subjected to much less supervision. Nor are they included in annual reporting figures from the Ministry of Justice. Using freedom of information requests Panorama has found - for a programme screened tonight - that at least 125 serious new crimes in the last year will not make it into official government figures.

Since they were first issued in 2001, the figures have consistently put the number of rapes, murders and other grave offences by Mappa criminals at around 70.

Last year's report stated that 79 offenders out of 12,800 criminals being monitored at the high-risk levels had committed further serious crimes after their release. But once level-one offenders were included, the total is likely to have been closer to 200.

Justice Minister Maria Eagle said the released figures are intended to inform the public about the successes in monitoring the highest risk criminals and that plans to release information about everyone monitored by the Mappa scheme - more than 50,000 in total - have been delayed owing to concerns over accuracy.