Web users in the UK will soon be able to watch any BBC television show for up to 30 days after its aired.

TV shows like Doctor Who are expected to be available for download later this year after the BBC Trust gave initial approval to the BBC's on-demand plans according to BBC News.

The corporation said it expects to launch its long-awaited iPlayer, a computer application which allows audiences to watch or download any programme from the last 7 days sometime in May following a 2 month consultation.

Once launched a programme will remain playable for 30 days after being downloaded or 7 days after being watched.

The BBC Trust, an independent body that replaced the corporation's governors at the beginning of 2007, said the on-demand plans - which also cover cable TV - were "likely to deliver significant public value".

But it agreed with broadcasting watchdog Ofcom, which said earlier this month that the iPlayer could have a "negative effect" on commercial rivals.

As a result, the trust has imposed several conditions on the BBC.

It wants the corporation to scale back plans to let downloaded "catch-up" episodes remain on users' hard drives for 13 weeks, suggesting that 30 days is enough.

When people record a programme at home "if they don't look at it within 48 hours, they don't look at it at all", he said.

But some shows will be able to remain on a viewer's computer beyond the standard 7-day window using a feature called series stacking.

Every episode of a "stacked" series would be made available until a week after transmission of the final instalment.

Trustees said the BBC needed to be clearer about which programmes would be offered on this service - but suggested "landmark" series "with a beginning and end", like Planet Earth or Doctor Who, should be eligible.

The trust also asked the BBC to explore ways of introducing parental controls to its on-demand services, as it is worried at the "heightened risk of children being exposed to post-watershed material".

Podcasts also came under scrutiny, with the Trust recommending that audio books and classical music be excluded from the BBC's download services.

The news will be a disappointment to the one million people who downloaded Beethoven's symphonies in a Radio 3 trial last year.