Two months ago Martyn Drake and his wife Jennifer, from Woking in Surrey, became so fed up with paying £139.50 for the BBC licence fee that they gave their television set away.

The couple still keep up with their BBC favourites, including Stephen Fry in America and Doctor Who, but now watch for free on a laptop computer. They are part of a growing band who legitimately avoid the licence fee by watching shows online an hour after they are broadcast live.

Since the iPlayer service began last Christmas, more than 220m requests have been made for programmes online. The service, backed by Mark Thompson, the director-general, as moving the BBC into a new medium, could, some claim, destroy it.

Don Foster, the Liberal Democrats’ culture spokesman, said: “We will see more people realising they can watch TV online without a licence. Funding for the BBC will decline.”

For Drake, a 32-year-old IT consultant, it no longer makes sense to impose an exceptional charge for the BBC. “There’s so much else out there with Sky, cable and other channels. It’s not fair that it all goes to one broadcaster.” Channel 4 and ITV are also available online.

Until now, refuseniks have risked prosecution. But those who watch “catchup” television are beyond the reach of Television Licensing (TVL), which enforces the law.

During the summer, it ran an advertising campaign warning office workers tempted to watch the Olympics on computers at work that their employers risked breaching the law.

But TVL is less eager to point out that shows watched after their first transmission are free. This is not stated on the licence or on warning letters.

Asked why the BBC did not make this clearer, a spokesman said: “Our issue is not to inform people of when they don’t need a licence. It’s to inform them when they need a licence.”

Despite the lack of publicity, awareness of the exemption is growing. Since August more than 185,000 people have joined a group on Facebook, the social networking site, calling for abolition of the licence fee on the grounds that it is a “legally enforced extortion racket”.

Many of the members, who range from students to families and pensioners, are using the iPlayer service to avoid paying the fee.

Liam Hancock, a 19-year-old student at Sheffield Hallam University, is typical. “The TV licence has got to go,” he wrote earlier this month. “You can’t have people watching it on TV and paying for it when you can wait and watch it for free.”

Some viewers are determined to enjoy a free service without abandoning a larger screen. Suellen Peskett and her husband Matt, from Dorking, Surrey, tried replacing their television with a 28in computer monitor in July. Suellen, a 29-year-old graphic designer, said: “It’s a great way of saving money.”

Although the couple abandoned the experiment after finding download speeds were too slow, they intend to try again as technology improves.

The problem for the BBC’s licence enforcers will be compounded from Thursday when BBC1 and 2 join the corporation’s minor channels in being broadcast live online.

While live shows do require a licence, broadband suppliers and politicians have warned that the corporation will not be able to catch evaders.

John Whittingdale, chairman of the Commons media select committee, said: “In the past they have tackled evasion by sending threatening letters. But now they can’t just come in and say that’s your television, where’s your licence? How will they know what you’ve used your computer for?”

The broadband companies who deliver the content say the technology does not enable the BBC to identify who is watching the shows, making enforcement virtually impossible.

When a viewer uses the iPlayer, the BBC receives a unique number called an internet protocol address [IP]. To get a person’s name and address it would need access to the databases held by broadband suppliers, but they are obliged to protect customers’ privacy.

A spokeswoman for Tiscali, a major broadband company, said: “The BBC would have no evidence of TV licence nonpayment and therefore would be asking us to pass over all customer data. It would be legally and morally unacceptable.”

A spokesman for BT said: “We don’t give that information [customer data] out. We don’t do it for file sharing and music piracy, and we wouldn’t do it for the BBC. They have to come to us with a court order and an IP address. Then and only then would we divulge an individual’s details.”

TVL admitted it has not developed any new detection methods to tackle the problem. A BBC spokesman said only a “very small” number of people had opted to cancel their licences and use the iPlayer instead. Additional reporting: Helen Brooks

Editor's choice

• Harry Potter to overtake James Bond as biggest franchise ever
• The 50 Best Movie Villains
• The 15 golden rules of theatre etiquette