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The iPlayer currently only allows viewers to watch programmes broadcast in the past week, which means people who watch an episode part-way through a series have no means of catching up with the show from the beginning.

But from 13 September, the BBC will start offering "series stacking" on a select number of shows, such as BBC One's Tess Of The D'Urbervilles and BBC Two's History of Climate Change.

There are limits

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to the series stacking service, however. Only 15% of the BBC's output is allowed to be offered for series stacking, as part of the BBC Trust's measures to prevent the service from harming commercial rivals. Channel 4's on-demand service, for example, offers every episode of a wide range of its shows - including popular comedies such as Drop The Dead Donkey and Spaced - for free.

The BBC's series stacking service will only initially be available for streaming - which offers poorer picture quality than the full download service because of its limited bit-rate. That said, the BBC is beginning to offer "high quality" 800kb/sec streams using the H.264 format on certain programmes.

Series stacking will be extended to full downloads and TV platforms (such as Virgin Media) "in due course".

"Series stacking marks a key development in our strategy to let audiences view our programmes whenever and wherever they want," claims BBC Vision's controller of multiplatform and portfolio, Simon Nelson.