Microsoft is facing a $200m (£126m) penalty after a US district court ruled that the company violated software patents held by Canadian software firm i4i.

The case centred on patents for the handling of XML code in Microsoft Word. Among the evidence presented by i4i were Microsoft emails which the plaintiffs said had demonstrated wilful infringement.

Microsoft has already vowed to appeal against the decision, and has argued that the i4i patents are invalid.

The ruling marks another entry in Microsoft's ongoing list of legal battles, most recently the settling of a patent suit with TomTom.

Patent infringement and sharing have been a big issue for Microsoft in recent years. The company has in some cases avoided going to court by signing patent sharing agreements with several companies, most notably Linux vendor Novell.