The US Attorney's Office for the Northern District of California is investigating the stock options backdating activities of computer and consumer electronics manufacturer Apple.

The federal action comes in the wake of the disclosure by Apple last month that it completed an internal investigation of options backdating and concluded that, although Apple chief executive officer Steve Jobs recommended some backdating of stock options for himself and other Apple employees, he wasn't aware of the accounting implications of those recommendations.

Luke Macaulay, a spokesman for the US Attorney's Office in San Francisco, confirmed the existence of the investigation, but did not provide additional details.

In a 29 December, 2006, filing with the US Securities and Exchange Commission (SEC), Apple disclosed that an October 2001 board meeting at which 7.5 million options for Jobs were granted never actually took place and that the options weren't officially granted until December of that year. Backdating the options grant date to October, though, gave Jobs and instant profit of about $20 million (£10.2m) because Apple stock was selling for less in October than it was in December.

Also in the filing, the Cupertino, California, company said it would take an $84 million (£42.9m) after-tax charge against earnings to correct stock options accounting errors in prior years. It acknowledged backdating about 6,400 stock option grants between 1997 and 2002.

"The board of directors is confident that the company has corrected the problems that led to the restatement, and it has complete confidence in Steve Jobs and the senior management team," said Al Gore, the former US vice president and the Apple director who chaired the special internal investigative committee.

Backdating stock options isn't necessarily illegal, but the practice must be disclosed and properly accounted for or it would violate federal regulations.