Violent crime - including knife crime - is increasing, say the new figures
A number of police forces in England and Wales have been undercounting some of the most serious violent crimes, the government has admitted.
It means figures for serious violent crimes rose by 22% compared to last year - rather than showing a fall as previous figures appeared to indicate.
The mistake happened when some crimes classed as "grievous bodily harm with intent" were recorded as less serious.
Figures say overall crime is down, and ministers say these can be trusted.
The government says it does not know how long the undercounting has been going on - leading to concerns that figures on violent crime may have been wrong for up to a decade.
A former Home Office crime consultant told the BBC the government had been "hiding behind" its changes in the crime counting rules.
Professor Marian Fitzgerald, a criminologist at the University of Kent's Crime and Justice Centre, said the long-term trend of increasing violent crime was now "catching up" with the government.
Let's be clear, this isn't crime that wasn't being recorded or wasn't being reported or wasn't being dealt with
Jacqui Smith, Home Secretary
The Conservatives said the new figures "fatally undermined" government claims that violent crime was in decline.
Shadow home secretary Dominic Grieve said: "They betray a government that is completely out of touch with what is going on, on our streets and in our communities."
Liberal Democrat home affairs spokesman Chris Huhne called for the Office of National Statistics - rather than the Home Office - to be given direct responsibility for crime figures.
But police minister Vernon Coaker said the "clarification" did not mean the government had lost faith in its crime statistics.
Home Secretary Jacqui Smith insisted all the crimes in question had been investigated by the police.
She told the BBC: "What the statisticians are clear about is that the increases in the most serious forms of violence have actually in terms of numbers been more than counteracted by the decreases in less serious violence."
FORCES WHICH RECOUNTED SERIOUS VIOLENCE
18 forces overall reported higher levels of serious violence
13 including above reported back
Metropolitan Police said it could not meet deadline
The error was discovered when at least 18 out of 43 forces in England and Wales were asked to re-examine their figures, following the realisation that some serious assaults were being recorded in a lower category of offence.
Crimes of "grievous bodily harm with intent" committed between April and June this year were being mistakenly recorded as lesser crimes.
When the figures were recounted using the correct classification, the official total showed serious violent crime had risen 22%.
Previous measures under the old rules had shown decreases every quarter of up to 15%.
Home Secretary Jacqui Smith on undercounting crime
Two of the most serious categories of knife crime showed big increases.
Attempted murders in England and Wales involving a knife between April and June this year were 28% up on last year.
And assaults causing grievous bodily harm (GBH) with intent rose by 29%.
However, the revised figures still show that overall crime and overall violent crime remain down on last year - a finding backed by the results of the British Crime Survey (BCS), also released on Thursday.
The BCS, which is a study of the experiences of victims, rather than police records, shows an overall decrease of 6% in the number of crimes compared to last year.
The Home Office has refused to name the police forces asked to resubmit their crime figures.
At least one of the 13 forces which complied - North Wales Police - said its crime figures were subsequently "audited, verified and found to be correct."
However experts say that other forces who did not submit figures, may have been making the same mistake.
Among them is the Metropolitan Police who has now confirmed a 44% increase in offences of GBH with Intent offences between April and June this year.
On Thursday the Met confirmed it would suspend issuing figures on serious crime until it had worked out the best way of presenting them.
Keith Bristow, from the Association of Chief Police Officers, said "most, if not all" police forces in England and Wales, plus British Transport Police, were to some extent misinterpreting the guidance on recording grievous bodily harm with intent.
Police figures are reflecting a long term trend increase in serious violence and the government are hiding behind changes in the counting rules to try to explain it away
But Professor Fitzgerald said that the government was aware of the long trend of serious violent crime which had been rising over "several decades"
She told the BBC: "It started to go up really quite steeply from the early 1990s.
"The problem this government has got is that when it came to power it dismissed out of hand the trends in police recorded crime which were a fairly good measure of serious violence
"It preferred instead to rely on the British Crime Survey which is very poor at picking up violence."
For good measure it has actually interfered with the police figures by keeping changing the ways in which they have been recorded.
"What's catching up with them now is the fact the police figures are reflecting that long term trend increase in serious violence. The government are hiding behind changes in the counting rules to try to explain it away."