Public confidence in crime statistics took a fresh blow after the Home Office admitted police have failed to accurately record serious violent crime.

Some forces may have been mis-reporting the figures for more than 10 years, officials said, as they announced an annual increase in violent attacks of one-fifth.

And months after the problem emerged, some forces were unable to provide updated figures, meaning the full extent of the most serious violent crime is not known.

Ministers said two thirds of the increase in serious assaults could be explained by officers putting them in a lower category of offence.

But only one third of serious knife attacks - which shot up nearly 30% - could be put down to reporting errors, officials said.

That revelation will call in to question the success of high profile attempts to tackle knife crimes and add to public fears of knife attacks on children following a string of high-profile deaths.

The Home Office asked 18 forces to re-examine their statistics after the problem emerged but only 13 were able to provide accurate information.

The Home Office refused to name the forces involved, but a Press Association investigation has established the Metropolitan Police was one of five which did not submit new data.

The country's largest force has suspended publishing some data as officials sort out the problem. Others who were not recording crime accurately include Humberside, Derbyshire, North Wales, Cambridgeshire and Thames Valley.

Senior officers have not complained publicly about how the rules were set out, but sources said they "required clarification". A senior Kent officer wrote to the Home Office to "express concern" at changes to the way crime figures are recorded and reported, a spokesman for the force said.