The five-year deadline for punishing securities fraud is shaping up to be a key point of contention as US prosecutors begin pressing criminal charges against corporate executives involved in backdating stock options.

More than 200 companies have been caught up in the scandal, in which firms altered the date of options grants to provide more favourable prices to key executives. But the vast majority of the mispricing occurred before mid-2002, when the Securities and Exchange Commission tightened the rules for reporting such grants.