3 Group, the mobile telecommunications operator owned by Hutchison Whampoa, has set limits on usage of its new "unlimited" mobile data plan in the UK.

3 UK has revealed that subscribers to its X-Series service can pay an additional £5 per month on top of their regular voice fees for "unlimited" access to Skype Voip (voice over IP), instant messaging and Internet browsing.

For £10 per month, subscribers get all that plus the ability to watch "unlimited" TV via a Slingbox plugged into their home broadband Internet connection. 3 sells the Slingbox for £99. Subscribers to this service can also use Orb software to remotely access content on their PC.

All those unlimited features come with limits, however. Subscribers are allowed 5,000 Skype minutes, 10,000 Windows Live Messenger messages, 80 hours using Orb and Slingbox TV and 1GB of Internet browsing per month. In addition, the included Skype service is restricted to calls to and from PCs or other Skype users on the 3 network, and customers must use Windows Live Messenger or Yahoo Messenger for instant messaging.

3 said that where unfair use is apparent, it will take "appropriate measures" based on individual cases.

Mobile operators have been criticised for pricing their data services based on how many megabytes or gigabytes users download. But operators like 3, that are beginning to offer flat-rate plans, are still under fire for placing limits on their unlimited plans.

Verizon Wireless in the US, for example, has been criticised for its $79.99 (£40) monthly "unlimited" data plan. On its website, Verizon notes that subscribers to the unlimited plans are authorised to browse the Internet and access email and intranets but they are not allowed to continuously stream audio or video, watch TV through devices like the Slingbox or receive automatic data feeds.

Some subscribers to the Verizon Wireless service complain that Verizon has cut off their subscriptions for over-use. Editors at a consumer complaints and reviews website detailed their experience this year after Verizon cancelled their account.

While placing limits on such data services may be disappointing to end users, the operators have a business reason for doing so. The operators are aware of the risk that a low-priced, unlimited offering could attract so many customers that their networks won't be able to support them all, said Daren Siddall, an analyst with Gartner, speaking after the introduction of X-Series.

3 introduced X-Series earlier this month and plans to open up the offering in the rest of its markets outside of the UK next year. The Nokia N73 is the only handset currently available supporting the offering but the Sony Ericsson W950i will follow in January.