A record rise in the number of complaints - 36% in one year - have been made to Elizabeth France, the data protection registrar, over the misuse of personal files held on computers, the agency revealed yesterday.

The 4,570 complaints is the largest number made to the agency. As a result it has overburdened agency staff who say they are facing "substantial backlogs" in handling them.

The surge has led to 587 visits by compliance staff to companies to check files for criminal offences. Another 11 search warrants were obtained to raid businesses run by private investigators who were illegally obtaining confidential information on people.

Some 145 cases went to court, with 130 businesses and organisations found guilty of illegally using or procuring the information.

Successful prosecutions detailed in the annual report include the Rugby Football Union, fined £1,500, R and B Consultancy Services in Swaffham, Norfolk, which was fined £4,000 for two offences, and Apex Computer Recruitment of Kingston, Surrey, which was fined £3,000.

Many of the cases involve misusing personal information for direct marketing or misusing consumer credit information held on computers.

In the report, Ms France warns the home secretary, Jack Straw, against overweening powers to obtain details of people's emails and to force internet providers to hand over encryption codes to fight cybercrime.

Mr Straw's plans would only be acceptable "provided that strict conditions were laid down in a national law in which an order or notice could be issued," she says.

As regards emails she calls for an amendment to the regulation of investigatory powers bill - now being challenged in the Lords - saying that interception must take place only after a judge or independent authority has ruled that there are grounds for doing so.

Ms France also says that membership organisations should make it clear to members how widely their details will be made available and for what purposes."