Police and prosecutors have been blamed for last-minute cancellation of one in every nine trials in West Yorkshire magistrates' courts.

According to a National Audit Office report, police and Crown Prosecution Service (CPS) errors wasted more than £55m in the year to March 2005.

The NAO criticised the CPS for:

l Switching lawyers during cases - a survey suggested the same prosecutor stayed throughout in just 15 per cent of cases

l Mislaying files

l Lack of preparation - with lawyers struggling because they received the files less than 24 hours in advance

l Failure to give priority to urgent cases - which meant necessary action was not taken in time for a hearing.

Police were blamed for leaving evidence out of files passed to the CPS - causing 35 per cent of ineffective hearings.

Last year, care worker Philip Tidswell, 43, of Queensbury, hanged himself after he was falsely accused of raping a mentally ill man. An inquest heard it took the CPS two months to decide there was no case to answer when it should have taken two weeks.

The trial of a man accused of causing the death of Ilkley woman Rachel Scantlebury by dangerous driving is yet to take place, after delays blamed on the CPS. The case, due last September, could not start because prosecutors did not serve evidence until the morning of the hearing.

A Brighouse dentist accused of a sex offence had his case dismissed because the CPS did not serve a vital document, and Bradford magistrates halted the trial of four football fans on public order charges after the prosecution failed to give defence lawyers information about alleged complaints of improper police conduct.

Edward Leigh, chairman of the Commons' public accounts committee which works closely with the NAO, said: "The innocent suffer avoidable duress, they are even kept in custody longer than necessary. And some guilty people must be getting away scot-free."

The NAO said the CPS had too many administrative staff and too few lawyers.

In all, 28 per cent, or 784,000 hearings, were delayed or ineffective in England and Wales, costing £173m a year.

NAO recommendations include making more prosecutor time available for review and preparation and prioritising some cases.