'We're very respectful of Microsoft, but when you're in the right you have to persevere.'

Microsoft plans to appeal.

'We are disappointed by the court's ruling,' spokesman Kevin Kutz said in a written statement. 'We believe the evidence clearly demonstrated that we do not infringe and that the i4i patent is invalid.'

The suit involves a patent i4i's founders obtained in 1998 that is the basis for a 'customised XML' tool the company supplies to drug and defence companies and other large corporations, said Owen.

XML is a specialised alphabet that can capture any kind of computer file as a regular text. It's designed to make computer data human-readable - and make it easier for one programme to load and process data created by another programme.

Judge Davis found that Microsoft was aware of i4i's patent and that there was enough evidence of Microsoft 'wilfully infringing' on the patent to issue the injunction, pending continuation of the case.

Investors shrugged off the news - perhaps in anticipation of a higher court overturning the ruling, which arose from the plaintiff-friendly Eastern Texas federal jurisdiction, Information Week reported.

Microsoft shares were up 1.6 percent to $23.50 in early trading in the US Wednesday.