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."