Ministers were accused today of creating "total confusion" over figures showing the number of high-risk offenders under strict state supervision.
Statistics from the Ministry of Justice appeared to show sharp falls in the number of violent and sexual offenders being monitored.
But instead of publishing a total of all the criminals falling under multi-agency public protection arrangements (Mappa), the figures for 2008/9 instead show a "snapshot" from one day.
A justice spokesman said the change would ensure consistency with the data on registered sex offenders.
But Harry Fletcher, assistant general secretary of the probation officers' union Napo said that in all likelihood the number of violent offenders had actually increased.
He said: "The government has changed the way in which it collates statistics on violent offenders referred to Mappa.
"This could give the impression that there has been a reduction in numbers and therefore less threat to the public.
"In reality, however, the number of violent offenders has almost certainly increased.
"The government claims it has changed the way it records in the interests of consistency.
"This may be the case but the effect has been total confusion and uncertainty about what the actual trends are."
Another change in the rules means some violent attacks are excluded from figures showing how many of the most dangerous criminals supervised under Mappa went on to commit alleged further crimes.
Some 48 offenders on the highest two levels of Mappa were last year charged with serious crimes - up to and including rape and murder.
That was a fall from 79 last year, but for the first time the numbers excluded those accused of "wounding with intent to cause grevious bodily harm".
Mappa panels, which include police, local councils and other government agencies, were set up to assess and manage the risks to the public from dangerous criminals after they leave prison.
The figures showed more than 1500 of the most dangerous supervised criminals, those on levels two and three of Mappa, were being returned to prison every day for breaching the terms of their release - or four every day.
Across all three levels, 12,425 violent, sexual and other dangerous offenders came under Mappa last year - a suggested fall of nearly 30%.
But unlike in previous years when ministers published a yearly total, that figure is the number being supervised on March 31.
Shadow justice minister Alan Duncan said: "Great credit must go to those involved in the difficult work of monitoring serious offenders and serious sex offenders in the community.
"However, 'snapshot' figures, which give the impression that the number of violent offenders referred to Mappa has sharply reduced, look like just another shameful attempt by this Government to mislead the public.
"Yet again the Government has failed to publish any data about lower level offenders, when they make up over two thirds of those covered by Mappa, and when we have good reason to believe they may commit more serious further offences than any other group. What have the Government got to hide?"Reuse content