The attorney for shareholders suing Apple executives and directors over stock options backdating said he will amend the federal lawsuit to reflect new information disclosed about its internal investigation of the matter.

A report filed 29 December by Apple with the US Securities and Exchange Commission (SEC) adds to the case against chief executive Steve Jobs and other Apple officials, said attorney Mark Molumphy, representing plaintiffs in a suit filed in US District Court for the Northern District of California prior to Apple's SEC filing.

In the probe report, Apple acknowledged Jobs' knowledge of the backdating, but absolved him of wrongdoing in the SEC report.

While a few Apple shareholders are named as plaintiffs, it is not a shareholder lawsuit, per se, but a "derivative" suit, Molumphy said. The shareholders are suing executives and directors of Apple on behalf of the company, alleging that the improper backdating has harmed the company financially.

Apple's internal investigation revealed that 6,428 stock-option grants had been improperly backdated between 1997 and 2002, but that while Jobs was aware of the grants, he did not financially benefit from any of them. The Apple filing noted that the company will take an additional after-tax charge against earnings of $84 million (£43.2m) in 2006 to account for the options.

"What makes the Apple case unique is that the backdating happened on such a wide scale and involved the top executives of the company, including the face of Apple, which is Steve Jobs," said Molumphy.

Apple's acknowledgement in the SEC filing is a "tacit admission" of a practice of backdating, he said. "It's been speculated about but it's never been part of an admission."

Stock-options backdating involves changing the date options are granted to a previous day, particularly to a date when share prices were lower than on the actual date stock options were granted. When executives exercise the option to buy the stock, they make more of a profit than if the grant dates were recorded accurately.

More than 120 companies have been investigated by the US’ SEC, Department of Justice and Internal Revenue Service over backdating practices. Some executives have been forced from their jobs and a few have been criminally charged.

Molumphy, a principal at San Francisco-based Cotchett, Pitre, Simon & McCarthy law firm said his firm has investigated backdating by other companies, but this is the first lawsuit the firm has filed on behalf of clients.

Apple was not available for comment.