Microsoft has stepped up its campaign against free and open-source software by claiming that the Linux operating system and other popular open-source products infringe more than 235 Microsoft patents.

Microsoft general counsel Brad Smith and licensing chief Horacio Gutierrez made the claims in an interview with Fortune magazine, published yesterday.

"This is not a case of some accidental, unknowing infringement," said Mr Gutierrez.

"There is an overwhelming number of patents being infringed."

In a subsequent interview with the US business magazine, Microsoft chief executive Steve Ballmer pulled back from saying that it would pursue individual users of open-source software for royalty payments.

Mr Gutierrez's comments have surprised some, as Microsoft appeared to be thawing in its attitude to open-source software in recent years.

Last November, it entered into an agreement with Novell, a leading provider of open-source software, which both sides said would improve interoperability between open-source software and Microsoft's products, as well as providing assurances over patent issues.

Con Hennessy, chief technology officer with Irish open-source services firm OpenApp, described the Microsoft comments as "sabre-rattling" designed to sell more software through its relationship with Novell.

"If you did a real analysis, a lot of the Microsoft patents would be thrown out," said Mr Hennessy. "They are either really obvious or you would find that someone else had created the same thing before it."

Mr Hennessy pointed out that Microsoft holds the patents in the US and said that software cannot currently be protected by European patents, so Microsoft's threats would have no impact in Europe.

Microsoft itself has fallen foul of US patent law recently.

In February, a San Diego court awarded $1.52 billion in damages to Alcatel-Lucent after it found that Microsoft had violated patents it holds related to MP3 digital music technology.