The release of Windows Vista may be delayed again, this time by the European Union. Anti-trust officials have warned that some of the software's security functions may be in breach of European regulations.

Jonathan Todd, EU competition spokesman, told journalists that it was up to Microsoft to ensure that it complied with EU anti-trust rules, and that open competition is the best way to improve software security.

"Such diversity and innovation could be at risk if Microsoft was allowed to foreclose the existing competition in computer security markets ... by bundling its own security products into its dominant operating system," he said.

But analysts are concerned by the moves, pointing to a lack of clarity and transparency in the process. Gary Barnett, research director at Ovum, said: "Microsoft should be scrutinised, but there needs to be a balance. We are calling for clarity: what is Microsoft being asked to do by the EU in the name of European software consumers, and is the company complying? One problem is the fact that the EU has no obligation to report its actions publicly."

Microsoft previously clashed with the EU in 2004 when a landmark ruling found that the software giant had abused its market dominance in audio-visual players. It was forced to remove its bundled Windows Media Player from the XP operating system.