Apple Computer's shares fell more than 6 per cent yesterday, a day after the computer maker warned it might have to restate its financial results because an internal investigation had found further irregularities relating to its granting of stock options.

The company said it had delayed filing its most recent quarterly report with the Securities and Exchange Commission (SEC) and its results from September 2002 onwards should not be relied on.

The news came a month after the maker of the popular iPod personal music player said it did not expect any material adjustment to its financial results as a result of the investigation.

Analysts expressed surprise at the company's reversal. "Clearly this is a negative development that reflects poorly on the integrity of the managers involved," said Richard Farmer, an analyst at Merrill Lynch.

Apple's inquiry had earlier uncovered irregularities relating to stock options grants, including one involving Steve Jobs, Apple's co-founder and chief executive. That grant was later cancelled, according to Apple.

US authorities are investigating more than 80 companies whose managers are suspected of "backdating" or manipulating the grant dates of stock options, to coincide with low points in the value of their companies' shares, without disclosing their actions to shareholders.

Apple has not indicated that it is the subject of any formal investigation.

Apple's announcement comes at a sensitive time for the company, which faces legal challenges to its iPod business model in Europe and increased pressure from Microsoft in the US.