3 is the first UK mobile network operator to offer its users unlimited Skype-to-Skype calls and messages without ever having to pay, as long as they have a compatible handset and a SIM card from the operator.

3 said that from May 1 there will be no data charges or top-up fees for either contract or pay-as-you-go customers who use Skype. It also said it will soon expand the offer to anyone with a compatible unlocked 3G handset in the UK.

3 UK offers a range of Skype compatible phones, including models from Samsung, Nokia, Sony Ericsson and LG. Users can check if their phone works with Skype on 3's website.

The mobile network already offers mobile phones that come with tight Skype integration, and subscribers use 1.5 million minutes of Skype calls every day, according to the operator.

The Skype push is all about 3 trying to gain market share, according to Mark Newman, chief research officer at market research company Informa Telecoms and Media.

The UK is an intensively competitive market. 3 needs a way to increase its market share, and hopes that the publicity this brings will help it gain new customers, he said.

"Skype wants to be every mobile operator's best friend, but its too early to say how successful it will be. Voice is still mobile operators' core business, and it is still 80 percent of their revenues," said Newman.

However, operators have more to gain by working with Skype than working against it, according to 3. Subscribers that use Skype are, for example, less likely to switch to another operator, and they also use more traditional voice minutes and send more messages by SMS, 3 said.