Apple is ramping up its assault on Microsoft by launching a version of its Safari internet browser that works on the dominant Windows operating system.

The move means that Safari will compete for the first time with Microsoft’s Internet Explorer on PCs that run on Microsoft Windows.

As part of a series of announcements made by Apple at its Worldwide Developers Conference in San Francisco, the group also said that it would invite outside software developers to build web-based applications for its upcoming iPhone, which will be launched in the US on June 29.

The move means that outside companies will be able to build software tools that link to the internet — such as spread sheet-type applications that could allow users to collaborate while travelling — that will work with the iPhone.

“There are whole companies building applications this way, like Salesforce.com and Google,” Steve Jobs, the Apple chief executive, said.

For the most part, however, Apple used the event to highlight the features that will be included in Leopard, the next version of its own operating system, Mac OS X, which is slated to be launched in October.

Analysts said that Safari had the potential to act as a Trojan horse, by introducing PC owners to Apple products, a path already taken by the iPod music player.

Unlike the iPod, however, Safari will face fierce competition from Microsoft’s Internet Explorer, which accounts for about 80 per cent of the browser market and Firefox, the opensource browser which has taken share from IE and now accounts for about 15 per cent.

Meanwhile, investors were disappointed that Apple did not have anything more radical up its sleeve, sending shares in the company down $4.30, or about 3.5 per cent in after-hours trading.

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