Ofcom has sounded off about the BBC's plans to offer its programming through on-demand downloads.

The watchdog is worried that the on-demand service will have a “negative effect” on commercial rivals, and that this will limit their investment, which would “not be in the long-term public interest”.

The comments were made as part of a market-impact assessment given to the BBC Trust, which is evaluating the service and public value it will have.

The BBC's on demand service will be supported by an application called iPlayer, or through NTL-Telewest's cable TV service, and will rival Channel 4's recently launched 4od.

Ofcom has estimated that the BBC's service could account for about four billion viewer and listener hours by 2011.

However, because Ofcom is also worried about the effect it might have about DVD rentals and sales, it has recommended that the BBC restrict the amount of time downloaded programmes can be kept, and shorten it from the current planned 13 weeks.