Microsoft is expected to mark its fightback against Apple’s fast-selling iPhone with the unveiling later today of a new version of its operating system for mobile phones.

Steve Ballmer, Microsoft’s chief executive, is to make an announcement relating to Windows Mobile at the CITA technology show in the U.S.

Analysts believe the group will unveil a new. more consumer-friendly version - 6.1 - of Windows for Mobile, the operating system that powers many mobile phone enabling subscribers to use popular Microsoft applications on the move.

The Microsoft system. which launched in 2001, has proved a success in North America. However it has done less well elsewhere.

Ben Wood of CCS Insight, said: “It has struggled beyond its home market and struggled to break outside of the business user market.”

Vodafone recently unveiled a specially built version of Windows Mobile 6.0 for several of its new Christmas handsets. Any new version, say analysts, could be modelled around this.

A recent report on The Boy Genius blog speculated that the new version would have a carousel-like menu system that would enable one-click access to popular functions.

Rival internet giants like Apple are putting increased pressure on Microsoft in the mobile sector.

The iPhone, which has already been snapped-up by more than 1 million Americans, is being marketed in large part on its aesthetic appeal and ease of use.

Google is also believed to be preparing to launch its own operating system for mobile phones, aimed at helping it to grab a chunk of the mobile advertising market.

Research by Informa Telecoms and Media forecasts that the market for advertising on mobile phones is set to be worth more than $11.3 billion a year in 2011.

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