Apple is planning to release version 10.5 of OS X on 26 October.

Codenamed Leopard, the latest instalment is the first new version of the operating system since the release of OS X Tiger in April 2005.

Server and client versions of Leopard will go on sale at 6pm local time at retail and online stores. The company is currently accepting pre-orders through the online Apple Store.

One of the software's key features is Time Machine, an archiving and back-up feature that allows users to restore files or file versions to a previous state.

The feature can undo changes to a file, restore a system to a working state after a crash or restore files that were accidentally deleted.

Time Machine is based on the ZFS file system released by Sun Microsystems under an open source licence.

Leopard will also have a new interface feature known as Stacks which frees up space on the desktop by organising files on the dock under a single icon which can then be scrolled through vertically.

The new operating system had been scheduled for release in June at Apple's annual Worldwide Developers' Conference, but was delayed by four months when Apple had to move engineers to the development of the iPhone.

Apple chief executive Steve Jobs took his trademark romantic view in announcing the new software, calling it the "best upgrade we've ever released".

Leopard is priced at $129. A server version costs $499 for a 10-client licence and $999 for unlimited clients.

Users who purchased a new Mac after 1 October will be eligible for a free copy of Leopard, although they will need to pay a $9.95 shipping fee.