Microsoft has been hit with a second guilty verdict - and heavy fines - for patent infringement in as many months.

Following the $388 million the software giant has been ordered to pay Uniloc over issues with an anti-piracy solution, a federal jury in Texas has now added $200 million to Microsoft's court case tab.

The jury ruled that the custom XML tagging features of Word 2003 and Word 2007 infringed on a patent from Canadian company i4i.

"We believe the evidence clearly demonstrated that we do not infringe and that the i4i patent is invalid", a Microsoft spokesman said in a statement, using the same defence they tried with the Unioc case.

"We believe this award of damages is legally and factually unsupported, so we will ask the court to overturn the verdict".