The BBC's Planet Earth was filmed in HDTV

The service is limited at the moment to shows made by BBC Worldwide and users must have an HD-ready TV and a Telewest TVDrive set-top box to receive it.

HDTV provides a sharper, clearer and more colourful image than the current standard television picture.

Sky intends to launch its HD service this year, offering film, sports and documentary channels.

It has already begun testing the equipment, taking HD cameras to sporting events including football and rugby union matches.

Still in infancy

Telewest plans to expand the ranch of programmes it can offer in HDTV, with the addition of HD films.

The BBC has trialled a number of HDTV programmes including the latest big-budget nature programme Planet Earth and dramas Bleak House and Rome.

The enhanced HD image is due to the way the picture is filmed, broadcast and displayed on HDTV sets, which use a greater number of pixels to display images than on a standard TV set.

HDTV is still very much in its infancy, with equipment such as HD-ready TV still an expensive prospect.

And capacity for high definition broadcasts on terrestrial television (Freeview) will be limited until digital switchover - between 2008 and 2012.