People who watch TV on mobile phones or computers could face a £1,000 fine unless they have a £126.50 TV licence.
An increasing number of TV shows and repeats are being shown on portable devices as well as standard TV sets.
But shows broadcast on any device at the same time as they appear on TV are governed by standard licensing laws, a TV Licensing spokesman said.
"Most who watch TV on a mobile phone or computer also own a TV and are already covered by an existing licence."
"But those who only use their computer, for example, to watch TV broadcasts will need to ensure they are properly licensed."
Live football matches
However, viewers do not require a TV licence if they only use mobile phones or computers to watch shows which aren't being broadcast "live", such as video-on-demand.
Mobile phone operators already broadcast live news channels and football matches to their customers.
An ongoing trial in Oxford, conducted by O2, has found that nearly 80% of people would subscribe to a mobile TV service.
Technology consultancy Strategy Analytics predicted that mobile firms would have about 50 million users of mobile TV by 2009, generating an estimated £3.5bn in revenue.
Meanwhile broadcasters including BBC, Sky and MTV plan to increase the number of programmes they make available via portable devices and broadband internet.
TV Licensing already gathers data from computer retailers and is "in touch with" mobile phone companies and retailers to ensure viewers have a licence as required.
"We have a database of 28 million addresses that shows us who does and does not have a current TV licence," the TV Licensing spokesman said.
"We have caught people watching TV on computers already."
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