The EU Competition Commissioner, Neelie Kroes, has warned Microsoft that its forthcoming Vista operating system could break European antitrust laws.

Ms Kroes wrote to Steve Ballmer, the chief executive of the software company, outlining potential problems with the new system, which will go on sale early next year to supersede Windows XP.

Jonathan Todd, a spokesman for Ms Kroes, said that regulators were worried that Vista’s new functions would limit consumers’ choice of software packages.

"We’re concerned about the possibility that the next Vista operating system will include various elements which are currently available separately, either from Microsoft or other companies," he said.

Vista will include an integrated internet search, a digital rights management programme that can be used to protect copyright, and software that will create fixed document formats similar to Adobe’s pdf.

Mr Todd said the EU’s letter was a response to Microsoft’s request that regulators set out any possible antitrust problems with Vista, which is Microsoft’s first major update to its flagship operating system since Windows XP was released in late 2001

The company is anxious to avoid a repeat of the legal battle provoked by Windows XP. It was fined €497 million (£342 million) by the EU in 2004 for breaching competition regulations.

Microsoft was accused of keeping secret technical details that would allow software companies to design Windows-compatible software and of killing off rival media players by bundling its own software, Windows Media Player, with every operating system it sold.

The company was ordered to divulge information about its software and to make a version of Windows XP that did not include Windows Media Player, but it still faces a fine of €2 million (£1.4 million) per day for what the EU says is its failure to comply with that ruling. Microsoft’s appeal against the fine is scheduled for the end of April.

Microsoft said that it could not comment on the contents of the EU’s letter because it has not yet received it. But it said that consumers were free to use a wide range of competitor products and Vista was designed to respect the choices that consumers make.

"Keeping the industry and regulators informed of our product development plans has been, and will remain, a priority," the company said in a statement. "We have worked hard to include partners and competitors in our planning so they can build products and services that work with Windows Vista."

Microsoft announced last week that the consumer version of Vista will be delayed until early next year, further extending the long gap between major Windows releases. A version for large business customers is due out in November.

Last month, a group of Microsoft rivals filed a complaint with EU regulators, alleging that its business practices threatened to deny real choice among competing software products.

The European Committee for Interoperable Systems – which includes IBM, Nokia, Sun Microsystems, RealNetworks and Oracle – said it was asking EU regulators to end practices that reinforced Microsoft’s existing monopolies and extended its market dominance into current and future product markets.

It would not say if Vista was mentioned in the complaint but did say that it had listed concerns about Microsoft’s Office software suite, which packages word processing, spreadsheet and office management tools.

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