A free 3G mobile broadband service has been launched in the UK, provided users agree to regularly watch a crop of video adverts.

Companies offering their ads through the service include Dell, Volvo, Xbox Kinect, and Clinique.

Called Samba, the service piggy-backs on the Three mobile broadband network, with subscribers paying £2.99 for a SIM that can work in a user's existing mobile broadband dongle device. A new dongle from Three costs users of the service £20.

Subscribers build up data credit when they watch video adverts offered by Samba, allowing them to go on the web where they like once they have built up a suitable credit.

Samba has banned access to porn sites and content it has deemed offensive.

Users are told their website visits may be tracked as another condition of using the service.

London-based founder of Samba Ben Atherton said: "With Samba you earn the credit watching ads at a time that is convenient to you and then have access when you need it."

To use the service, subscribers must install a web browser plug-in for their laptop or netbook, or an app on their iPad. This software directs them to a selection of adverts that build up their data credit.

Samba says watching 2.5 minutes of adverts a day can build up enough credit to download over 500 megabytes of data on the Three mobile broadband network.

Three currently charges subscribers £15 a month to get 15 gigabytes of data, although there are cheaper packages than that for less monthly data allowances.

If Samba users buy an item from one of the firm's advertising partners they gain additional credit. There is also an option to buy data with cash top-ups.