American computer company Hewlett-Packard has admitted using legally questionable methods to spy on one of its own directors, in a scandal that has prompted investigations by law enforcement authorities.

HP disclosed yesterday that it had become increasingly frustrated by damaging leaks of sensitive boardroom discussions in the run-up to last year's departure of its chief executive, Carly Fiorina.

In an effort to pin down the source of media reports, it hired private investigators, who identified the culprit as George Keyworth, a long-serving non-executive who was once President Ronald Reagan's scientific adviser.

Mr Keyworth was asked to resign in May, but he refused, saying he had been elected by shareholders. A fellow director, Thomas Perkins, quit in protest at the way the inquiries had been carried out.

The saga was revealed yesterday in a filing with the United States securities and exchange commission, which is investigating whether HP broke its rules by failing initially to give a detailed reason for Mr Perkins's departure.

In the document, HP revealed that California's attorney-general had requested information "concerning the processes employed in the investigations into the leaks" and was examining the affair. HP said its investigators had used a method known as "pretexting" - disguising their true identity - to obtain Mr Keyworth's phone records.

The law surrounding pretexting differs from state to state in America. HP insisted it was "not generally unlawful" in California, except when used to obtain financial records.

However, the company added that its legal advisers "could not confirm" that the methods used by its investigators "complied in all respects with applicable law".

The bust-up dates back to a story in the Wall Street Journal in January last year which reported that HP's board was unhappy with the company's personal computer business and wanted Ms Fiorina to hand over some of her responsibilities to fellow executives. The newspaper said the idea had been raised at a three-day board retreat at San Francisco's Park Hyatt hotel.

Although the story was accurate and Ms Fiorina was dismissed two weeks later, occasional unauthorised reports continued. Following another board awayday, a source disclosed to the website CNET: "By the time the lectures were done at 10pm, we were pooped and went to bed."

HP's chairman, Patricia Dunn, was furious and began a quest to track down the source. Mr Keyworth was confronted with evidence about his conduct in May.

Corporate governance experts say confidentiality is a thorny issue for directors. Board members have a fiduciary responsibility to keep price-sensitive information to themselves - but leaks irrelevant to the share price are a grey area. The use of private investigators in America is relatively commonplace. The US Association of Professional Investigators say that more than 65,000 practitioners hold licences nationwide. Insurance companies routinely use them to look into suspicious claims.

A spokesman for the association, Fred Ritz, said: "Most investigators, without a court order, generally would not pretext for somebody's phone records."

HP's shares slipped by just over 1% on the New York Stock Exchange yesterday. The company said Mr Keyworth would not be nominated for another term as a director when the current one expires next year, although it declined to comment further.

Ms Fiorina plans to disclose her own version of the events leading up to her downfall in a book to be published next month.