Microsoft has clamped down on UK firms selling pirate versions of its software.

An investigation by the software giant uncovered 21 UK resellers that had loaded illegal copies of Microsoft software onto hard drives of PCs they were selling.

The investigation also caught four traders using auction site eBay to flog the illegal software.

Michala Wardell, head of anti-piracy at Microsoft UK, told Computer Weekly that many people are unaware that they are not being sold the genuine product.

"We attempt to deal with most offenders ourselves, but where sellers persist in illegal activities, we call in UK trading standards officers," she said.

Wardell revealed that London, Huddersfield, Warrington, Manchester and Bristol were among the areas the pirates were trading in, leading Microsoft to believe student populations are driving the demand for pirated software.

She also said that of the companies caught selling pirate software, many said they were unaware what they were doing was illegal or that the crime had been committed by an employee without the firm's knowledge.

"At a time when UK resellers are looking to expand their business post-recession, piracy not only impedes growth, but puts local hard-working computer shops out of business,"

Most of those caught in the operation either claimed the illegal activities were carried out by rogue employees or that they were not aware that what they were doing was illegal, said Wardell.

See also: 33% of Brits think using pirated software is acceptable