By Darren Waters
BBC News technology staff
The pirating of a Doctor Who episode on the net before it is even broadcast has put TV downloading into the spotlight.
Doctor Who hits the net before the airwaves
But how big a problem is this for television broadcasters?
In 1999 Napster was launched, sparking a revolution in the music industry that is not even close to ending.
The availability of tracks online transformed the way consumers wanted to access their music and the industry spent years and millions of dollars trying to catch up with the pirates.
For the last couple of years TV programmes have been available online for download and there is the suggestion that if broadcasters do not accelerate their online plans they too could be hit by the "Napster effect".
These are not programmes sanctioned for download by broadcasters - in fact, apart from a few experiments, TV companies have resolutely failed to embrace downloading.
Desperate Housewives has proved a popular download
But where the corporations have hesitated, individuals have rushed in to provide a "service".
Every major US television series is available for download as a video file - you just have to know where to look - and an increasing number of British TV shows are also now available.
The Simpsons, Desperate Housewives, Joey, The West Wing, The Sopranos - all are just a click of a mouse button away.
EastEnders, Hollyoaks and old episodes of classic comedy programmes can also be downloaded for viewing on a PC or portable media device.
Recent research suggested that the UK was the number one country for downloading TV programmes from the net.
Almost a fifth of all TV downloading was by UK net users.
Web tracking company Envisional said a typical episode of a show like 24 was downloaded by about 100,000 people globally.
The programmes are encoded into a computer file by individuals who make that file freely available for download by other users.
Envisional said 70% of people were using file-sharing program BitTorrent to upload and download TV content.
BitTorrent works by breaking a file up into fragments and then distributing the data over multiple users - it means large files - such as video content - can be shared quickly and easily.
Two years ago I asked a number of media companies if they were aware of BitTorrent. Few were - but today I would suggest they have all now heard of BitTorrent.
There are a number of websites which provide links to the files for users to download the BitTorrent file but these are becoming scarce as media companies try to shut them down.
But there is a proliferation of private online groups, which are based around shared interests and in some cases solely around sharing TV content.
The Doctor Who episode first appeared online in a private group dedicated to the time traveller. Word of the episode's existence spread across the internet via blogs and news sites and soon other peer to peer networks were offering links to the programme.
Media companies say their copyright is being infringed by those who copy the TV programmes, and again when a user downloads the file, even when it is for personal use.
Fans of TV downloading argue that it is no different from recording a programme on a VHS tape and then lending it to a friend.
Some fans download entire series and copy them to DVDs
But with some people downloading entire series of a show and then copying it to DVD, media firms fear they will lose valuable sources of revenue.
In the UK the situation is muddied further - often many of the US programmes have yet to air in the UK.
With broadcasters such as Five paying millions of pounds for the rights to series such as Joey and CSI: NY it is obvious they will become concerned if people are watching online rather than watching their transmissions.
"It's now as easy to download a pirate TV show as it is to programme a VCR," said Ben Coppin from Envisional.
That is not strictly true.
In the case of the Doctor Who episode, the downloaded file will not play directly on PCs using Windows Media Player or other popular players, such as Real Player.
There are many different file formats and not all media players will be able to cope with them.
But that is the downside of a "service" which is growing as broadband penetration takes off and download speeds increase.
The methods for finding and downloading the programmes are also becoming more sophisticated.
There are now computer programs which can be controlled much like a video recorder or PVR such as Sky Plus - you select your favourite shows and the program takes care of the rest.
For TV broadcasters it is a problem that is only going to get worse.
If the music industry is a model, then we can expect lawsuits to tackle the worst offenders, but the bulk of downloading by the hardcore will remain untouched.
The BBC has trialled a download service for some of its TV content, but there is no indication yet as to when the Interactive Media Player will launch.
Other broadcasters are probably hurrying their own plans because until they launch, there will be plenty of individuals willing to fill that hole.