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

The Samba 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. If they haven't got a dongle they have to pay £20 for one.

When the subscriber watches video adverts offered by Samba they build up data credit, to allow them to go on the web where they like. Samba has banned access to porn sites and content it has deemed "offensive", which may be hard to quantify for some users of the service.

Another condition of using the service is that users are told their website visits may be tracked. This type of data is useful for marketing firms and advertisers, and tends to be sold on by the organisations collecting it.

London-based Samba was founded by Ben Atherton, who comes from a marketing background. He 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.

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