More than 1,000 directors of US companies have benefited from backdating stock options to boost their pay, according to new research that is set to open another front in a scandal that is spreading rapidly across corporate America.

The study by three academics – to be published on Monday – is the first to provide evidence that options of outside directors were backdated in the same way as the ones awarded to chief executives. It could prompt regulators and companies to widen their probes to include backdating by directors.