T-Mobile UK has shifted its 3G pricing to a flat-rate scheme, slashing its prices by more than half and scrapping all its per-megabyte rates.

The company is the first UK operator to move exclusively to flat-rate plans, which could make 3G a far more attractive proposition to businesses.

But with the freedom comes significant restrictions designed to protect T-Mobile's other mobile services - for instance, instant messaging and Internet telephony are both banned.

While the new Web 'n' Walk flat-rate service allows users to browse any Web site they choose, all versions of the service carry serious limitations. The most permissive, the professional tariff, limits UK data use to 2GB per month. If users are detected using VoIP or IP instant messaging, the consequences could be serious.

"If use of either or both of these services is detected, T-Mobile may terminate all contracts with the customer and disconnect any SIM cards and/or Web 'n' Walk cards from the T-Mobile network," read the company's terms of service.

The consumer-oriented Flext and Relax packages, designed for smartphones, are still more draconian, banning users from going online with a laptop, as well as from viewing streaming audio and video, peer to peer file-sharing services and video downloads. These and other services are or may be offered by T-Mobile itself.

Still, the packages represent a significant shift in the UK's market for 3G data services. The professional tariff offers an 18- or 24-month contract, bundling the cost of a laptop data card, for £19.99 per month including VAT. This compares with T-Mobile's previous flat-rate charge of £45 per month, set deliberately high to avoid cannibalising customers from more lucrative per-megabyte plans.

The data card is quad-band GSM, enabling it to run on 1900GHz networks in the US, and supports Wi-Fi as well as the high-speed HSDPA (High-Speed Downlink Packet Access) flavour of 3G. T-Mobile is throwing in 12 months of free Wi-Fi access to sweeten the deal.

T-Mobile will switch on HSDPA in the UK late in the third quarter of this year, and will offer a free upgrade to the service when it becomes available.

Some operators already allow VoIP on their networks, such as Germany's E-Plus, which offers a device bundled with a Skype client. Skype has an agreement with Hutchison 3G to bundle its service with Hutchison operators in Europe and Asia-Pacific. Hutchison owns the UK's 3 service.