Figures about some of the most serious violent crimes have been miscounted, it has emerged.

The Home Office revealed on Thursday that 13 police forces have been asked to re-examine their figures on crimes such as serious assault, murder, attempted murder and manslaughter.

Officials believe the undercounting may have gone on for more than 10 years and could be responsible for sharp rises in some crime figures this year, although ministers have insisted that public confidence in official figures should not be affected.

Attempted murders in England and Wales involving a knife between April and June this year were 28 per cent up on last year, while assaults causing grievous bodily harm with intent rose by 29 per cent.

It is thought that the forces concerned – who the Home Office have refused to identify – were putting the crimes in a lower category of offence.

Paul Wiles, chief scientific adviser to the Home Office, said at least two-thirds of the increase in grievous bodily harm was as a result of the under-reporting. “As to the other one third, we simply don't have clear evidence at the moment,” he said.

The problem arose after offence categories were revised from April 2008.

Home secretary Jacqui Smith said: “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 seven per cent - almost 18,000 fewer violent crimes.”

Police minister Vernon Coaker insisted that he was not embarrassed by the revelation.

“I would be embarrassed if I wasn't coming forward and not trying to draw to public attention something which is important,” he said. “I want the statistics to be as accurate as possible. We have every confidence in the crime statistics.”

Shadow home secretary Dominic Grieve said Labour’s target-driven approach had “simply been to manipulate the statistics”. He added: “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.

“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.”