Some police forces have been failing to record serious violent crime properly, the Home Office said today, as it reported an increase of 22 per cent this year.
The category includes serious assault, murder, attempted murder and manslaughter.
Officials said 13 forces were asked to re-examine their figures after they discovered some serious assaults were being recorded in a lower category of offence.
They admitted the under-counting could have been going on for more than 10 years.
Two of the most serious categories of knife crime also shot up after the miscounting was revealed.
Attempted murders in England and Wales involving a knife between April and June this year were 28 per cent up on last year.
Assaults causing grievous bodily harm with intent rose by 29 per cent.
The forces involved had been putting more serious crimes in to a lower category of offence, officials said.
The Home Office refused to name the forces involved. Police Minister Vernon Coaker said the "clarification" did not mean the government had lost faith in crime statistics.
Home Secretary Jacqui Smith said more work needs to be done to tackle violence, including knife crime.
She said news that 13 forces under-reported some of the most serious crimes should not dent public confidence in the crime figures.
Ms Smith said the Government will continue to work to find ways to record the most accurate crime information.
She said: "Last year we reduced police targets to give them greater flexibility to deal with local crime problems and to prioritise tackling the most serious violence.
"We revised offence categories for recording the most serious violence and clarified our guidance to police forces.
"This change means that this quarter's figures are not comparable with last year's.
"However, for most types of violence where the guidance hasn't changed - like homicide, robbery and death by driving - the numbers are down since last year.
"And the overall number of violent crimes is down too - by 7 per cent - almost 18,000 fewer violent crimes.
"Although it represents less than 1% of recorded crime, reducing serious violence will always be a priority for us."
Chief Constable Keith Bristow, of Warwickshire Police, said the police must respond to worrying trends in the figures.
He said accurate reporting of all types of crime is essential to help police respond and boost public confidence.
Mr Bristow said: "The impact of changes made to the way in which some crime is recorded will make uncomfortable reading.
"Apparent increases in some categories of violent crime are strongly influenced by these counting amendments which were introduced by the Home Office in April 2008.
"Acpo supported the amendments to the counting arrangements as these changes will improve our understanding of neighbourhoods affected by crime.
"The value of crime statistics lies in their ability to provide accurate information about what is happening in neighbourhoods.
"This allows the police service to take action where it is needed.
"Some categories of crime have historically suffered from under-reporting by the public, such as sexual assault and domestic abuse.
"The police service continues to actively encourage the public to report these types of offences.
"While the statistics published today show that crime continues to fall year-on-year, the public is rightly concerned about aspects of violent crime and disorder.
"Effective policing is built on trust and confidence and we want to assure the public that the police service is committed to working with them and other agencies to protect them from harm."
Mr Bristow is the Association of Chief Police Officers (Acpo) lead on violence and public protection.
Shadow home secretary Dominic Grieve said: "These figures fatally undermine Government spin that violent crime was getting better and betray a Government that is completely out of touch with what is going on, on our streets and in our communities.
"Labour's target-driven approach has simply been to manipulate the statistics.
"They should now face up to the reality of their failure and realise that if you can't count a problem, you can't combat it.
"In any event, serious violent crime would still have increased before miscounting was revealed.
"After 10 years the Government have proved themselves incapable of delivering crime statistics that reflect what is going on and which the public can have confidence in - a lot of this is due to their obsession with spin."Reuse content