Mobile operator 3 has announced the first flat-rate mobile Internet plan providing unlimited use for a monthly fee.

But not all is as it seems, with the flat-rate fee coming on top the usual monthly subscription fee, the "unlimited" usage held to limits that the company has yet to announce, and high-bandwidth applications such as video likely to attract a further fee.

Under the new service, called X-Series, customers will be able to access online applications from their phones without worrying about the per-unit fees in most contracts at the moment. The company, owned by Hutchison Whampoa, announced the new service alongside a range of big Net names including Skype, Yahoo, Google, eBay, Microsoft, Nokia and Sony Ericsson.

X-Series will launch first in the UK on 1 December, and then in 3’s other markets, including Italy, Sweden, Hong Kong and Australia, early next year.

3 hasn't announced pricing for the service yet, but the flat fee for unlimited Internet access will apparently come on top of the usual monthly phone subscription. Customers will initially be able to choose from two phones for the service, the Nokia N73 and the Sony Ericsson W950i.

They will be able to make unlimited calls from their phones using the Skype's VoIP service. Most mobile operators have been reluctant to enable Skype because it could compete with their own voice offerings. Skype has previously been available only on mobile phones running a Microsoft operating system.

Subscribers will also be able to use instant messaging from Yahoo and Microsoft, search using Yahoo or Google, and shop for items on eBay.

The service is also only unlimited in the ISP sense of "unlimited" - there will be "fair usage" limits that have not yet been disclosed. Similar limits in the US have come under criticism because they bar users from accessing high-bandwidth services such as online video.

Three is apparently considering charging an additional fee for customers who use high-bandwidth applications through Slingbox and Orb. The Slingbox is a device that you connect to your cable box or satellite receiver which allows you to watch any of the programs remotely on a PC or phone. Orb allows users to access content from their PCs remotely. Orb designed a user interface for the X-Series handsets, so customers can use their phones to access files from their computers.

Original reporting by IDG News Service