There’s something strange about Heroes. NBC’s autumn hit in America has everything a drama needs to be tagged “the next Lost”: a gorgeous young cast, hyped-up moral issues, dead loved ones who need avenging and a confusing puzzle that must be solved to avert global catastrophe. On its first night, it pulled in 14.3m viewers, the highest rating for an NBC drama debut in five years and a notable performance in a season mired in failure. Its success wasn’t entirely suprising: superheroes always do well when America’s paranoia is at its greatest. Add to that characters such as a Texan cheerleader who can heal from any injury, a heroin-addicted New York artist who can paint the future and a smouldering Indian scientist, all facing off against a serial killer who plans to nuke New York, and you have sure-fire talk-about TV.

What’s strange about Heroes is that it’s not just the most talked-about show in America. It’s huge over here. In the chat rooms on the Internet Movie Database, almost half of the Heroes comments are posted from the UK. Our TV gossip sites are alive with conversation about plot twists in episode nine. Yet Heroes doesn’t reach our screens until next year. The Sci Fi channel launches it in February; BBC2 will screen it in the summer. The series is too new for a US DVD release, so how are all these people watching it? Have British youths developed their own mysterious superpower, the ability to watch American television across the ocean? Well, no. According to the online piracy analyst Envisional, our main superpower is nicking stuff off the internet.

Envisional’s report finds that Brits lead the world in illegal programme downloads — making 18% of all transactions on “peer-to-peer” websites such as SuperTorrents and TV Links, which connect downloaders to one another. It’s a barter-based economy: you go onto the site having recorded a programme on your state-of-the-art laptop or hard-disk recorder, and swap it for other programmes; most are available pretty much in real time. One episode of Six Feet Under was ready to view less than half an hour after transmission. The figures aren’t earth-shatteringly huge, but they are building: Envisional estimates that series four of 24, for instance, was downloaded roughly 95,000 times per episode.

We have been here before. Napster launched in 1999 as a music version of the above, and had at least 27m users two years later, when it was closed down by record-company lawsuits. “What we are concerned about is making the same mistake the music industry has made,” says Rod Henwood, new-business director at Channel 4. “It let the illegal sector grow, and that sector still commands almost 90% of online music consumption.” Henwood says the risks are high: in 2005, music-CD sales slipped by 3% as consumers migrated to the net, with teenagers moving fastest.

It looks as if the same thing is about to happen to the humble television set.

Channel 4’s forecasters estimate that between 10% and 30% of today’s viewers will end up watching most of their TV programmes as downloads on computers or iPods within the next few years. A new generation is on the way, one for which a TV set is just one way to access “product”.

It is already affecting broadcasters’ scheduling plans. In the United States, ABC starts the new segment of Lost on February 7. Although BSkyB refuses to confirm its start date, the industry expects it to air episodes within days of their American screening — in part to head off the peer-to-peer community. BSkyB is paying £975,000 per episode for Lost, and is relying on fans to take up subscriptions to make it pay; free file-sharing isn’t in the business plan.

British broadcasters are trying to get in there first. Channel 4 launched 4oD, a sort of TV iTunes, last week. It hopes that 4oD, with an archive of 500 programmes and two new shows per week to rent or purchase, will pull downloaders into the legal sector. In a trial on cable last month, 1.5m downloads were made in less than four weeks. BSkyB, BT and the BBC are considering similar services.

According to Edward Waller, editor of the TV trade bible C21, this may alter programming for ever. Trials of a 4oD-style service by the US network ABC found that scripted drama performed best, while reality shows were largely shunned. One downside is that, as viewers have to elect to watch the programmes, rather than perhaps stumbling on them, every show will live or die by ratings and off-screen marketing. Unfashionable areas such as news and documentaries are liable to suffer, as is the last great communal experience: chatting about yesterday’s telly with your colleagues.

One of the great triumphs of Saturday-evening television recently has been that shows such as The X Factor and Doctor Who have proved genuine family viewing. In many households, 5-7pm on Saturday is the only point in the week when everyone spends time in the same room. Downloading is likely to cut that last bond. The Heroes may save the planet, but they are helping to make the world a lonelier place.