In its market-impact assessment, the media regulator advised the BBC to remove or reduce certain features of its iPlayer in order to limit its effect on the market.
Ofcom said the "series stacking" option, which enables users to automatically download and store an entire series of programmes, "would make the service a more direct substitute for commercial services".
And this market impact would be "significantly increased" by the ability to store downloaded programmes for up to 13 weeks. "Unless the Public Value Assessment provides strong reasons for taking a different course, the 13-week strong window should be removed or substantially reduced," Ofcom said.
Channel 4, ITV and Sky all refused to comment officially on the findings. However, a Sky source said: "The key question is whether the BBC Trust chooses to heed Ofcom's findings or to act as a rubber-stamp for the application."