BBC shows can be downloaded for free on new iPlayer

Computer users need never worry about missing their favourite BBC shows again following the launch next month of the iPlayer service.

Four hundred hours of programmes per week will be available free for downloading up to seven days following broadcast, after which they will remain watchable for a further 30 days before deleting themselves.

The corporation describes the development as its biggest since the introduction of colour TV.

However the main drawback initially will be that downloaded shows such as Doctor Who must be viewed at the computer screen rather than the television, unless the two are linked.

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The programmes will not be copiable on to DVD thanks to anti-piracy technology.

And it will take as long to download a show as it will to watch it - tying up the computer for long periods.

An hour-long programme will initially take 60 minutes to download when the service launches on July 27.

However, rather than having to wait, viewers will be able to stream programmes live to their computers later this year and will also be able to "stack" up episodes of certain series without them being deleted until some time after the series is over.

The corporation eventually wants to make the free service available on TVs as well as computers, with plans to launch the iPlayer on cable later in the year followed by other TV platforms and eventually mobile phones.

Access to the service will be free, with the necessary information downloaded from the BBC website. Owing to rights issues, the iPlayer will not be able to offer some sports events, foreign shows and music programming.

The corporation has denied fears that the service will make it easier for children to access unsuitable content by claiming there will be clear labelling and "PIN protection" on programmes.

But there are still concerns that the iPlayer could massively slow down other Internet services and could lead to increased piracy of BBC shows if the anti-copying technology is circumvented.

The iPlayer, and other services from ITV and Channel 4, are the latest responses to the decline of traditional TV viewing, with consumers expecting to choose when to watch programmes.

BBC director general Mark Thompson said: "It is at least as big a redefinition of what TV can be, what radio can be, and what broadcasting can be, as colour Television was 40 years ago."

The BBC director of future media and technology Ashley Highfield the BBC is hoping to attract 1million users in the first year of the broadband TV service, with an initial target of 500,000 in the first six months.

The BBC's website already attracts 16million unique users per month.

There have also been discussions-with other broadcasters about teaming up for a combined free download service of the best of UK content.

While many broadcasters are offering download services, digital TV platforms are also operating a new breed of video recorders.

Sky+ for satellite viewers already has more than 2million users and Freeview Playback, a similar service for digital terrestrial viewers, is also preparing to launch.