Apple, the computer maker, today shocked its army of fans by unveiling software that will let its newest Macintosh machines run on Windows, the operating system developed by arch-rival Microsoft.

The watershed decision could help Apple to dramatically expand its share of the computer market.

The company has already seen sales of its computers boosted by the massive popularity of its iPod music player, dubbed the "halo effect" by analysts. But more than 90 per cent of home machines still run on Windows.

Shares in Apple rose as much as 6 per cent on the news, touching an intraday high of $65.60 in New York.

The company said a trial version of the software, called Boot Camp, will be available as a free download from http://www.apple.com/macosx/bootcamp/. The program will also be built into Leopard, the next version of Apple's own operating system, which is due to be unveiled in August.

Boot Camp will work on the recently unveiled new generation of super-fast Apple computers, the first to use processors built by Intel, the world's largest chip maker. Apple is eventually expected to build all of its computers to run on Intel chips.

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