Apple has abandoned full .Mac syncing support for Mac OS X 10.3.

Writing in a technical note the company warns .Mac users that Mac OS X 10.4.10 or later is required in order to ensure "uninterrupted" service for sync operations, particularly calendar sharing.

The company says the move is in order to support new enhancements to .Mac which are set to be made available when Leopard ships. "These changes will help enable .Mac Sync to do more than ever before," Apple promises.

"If you use Mac OS X 10.3.9 or earlier, be aware that in order to accommodate new enhancements, .Mac Sync will no longer support syncing calendars between a Mac OS X 10.3.x-based Mac and a Mac OS X 10.4.x-based Mac. Syncing calendars between two Mac OS X 10.3.x-based Macs is unaffected," the company warns.

The .Mac upgrade began on 19 October and is ongoing. The company warns that users attempting to perform a sync operation may encounter a dialogue asking: "What would you like to do for this first sync?"

The company says: "If you have received this dialogue, and have not recently reset your data on .Mac or registered your computer for syncing with .Mac, you can safely select to Merge data on your computer and .Mac. You may have to perform this step once on each computer that is syncing with .Mac."