The list of companies, which range from small- and medium-enterprises to larger firms employing hundreds of staff, had come from tip-offs to BSA's software piracy reporting hotline.

BSA's regional senior director of anti-piracy, Mr Tarun Sawney, said companies that get the letters 'have every right to say no', and it has no immediate plans to take any legal action against them.

He said many of the companies on the list may not know they are using illegal software, and thus, BSA has decided to take a softer rather than a harsher line against them.

'In many cases, the infringing software is due to employees taking unauthorised shortcuts, so this will allow them to get a good idea of where they stand,' he said.

The BSA will not act against audited companies, even if unauthorised software is discovered. But for those that say no, they face the risk of legal action if evidence of wilful infringements surfaces later.

The BSA has, in fact, obtained evidence including signed affidavits from informants about unauthorised software use by more than 20 companies. The BSA pays informants if their evidence results in successful recourse against companies using unauthorised software.

The BSA usually settles out-of-court with the companies for damages, legal costs, and an undertaking to use legit software, but it has also previously worked with the authorities to raid companies that flout the law.

Companies that use unauthorised software can be fined up to $20,000 under the Copyright Act. Rights owners like BSA members Microsoft and Apple can also sue for up to $200,000.