Microsoft has revealed that it will be making its mobile phone app store, Windows Marketplace for Mobile, available to owners of handsets running Windows Mobile versions 6.0 and 6.1.

When Microsoft initially announced the app store, the company said it would be available only on phones running Windows Mobile 6.5, which is due later this year.

It has now said older handsets will be able to access some of the 20,000 apps in the store, which is expects to be open by the end of the year.

However, not all of those applications will appear in the store immediately. The Marketplace won't aggregate all available applications, only those that Microsoft has tested and guarantees with a money-back offer.

Microsoft said it will start accepting applications for its Windows Marketplace for Mobile on July 27.

Some recent reports have suggested that around 600 applications will be available at launch. While a company spokesman confirmed that number has been given as a preliminary estimate, he said it's too early to know how many apps will be in the store at launch, since developers haven't yet begun to submit their programs.

Microsoft is playing catch-up to Apple, which popularised the concept of downloading phone applications with its iTunes app store for the iPhone. Apple recently said that 1.5 billion iPhone applications have been downloaded in the year the store has been open.

Others have already followed Apple's lead, with application stores available for Android and BlackBerry devices, as well as the new Palm Pre.

Microsoft is competing for developers with those phone makers, although many may build applications for multiple platforms. While developers may be attracted by the millions of Windows Mobile devices in use around the globe, those phones have considerably less buzz compared to the iPhone and other popular phones.

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See also: Microsoft bans Windows Mobile VoIP apps