The BBC wants to run a permanent Freeview hi-def pilot channel using existing capacity before analogue switch-off.

The BBC HD offering would run from 2am to 6am while other channels such as BBC Four and BBC Parliament are off-air.

The corporation hopes the hi-def teaser will generate enough pressure on Ofcom to give it space for a prime-time channel after analogue switch-off in 2012 - just in time for the Olympics.

The new Freeview scheme was revealed as the BBC Trust published full details of the BBC's HD channel proposal for its Public Value Test.

Satellite, cable and possibly internet viewers will be able to watch a daily hi-def channel from 3pm to midnight, as part of the main proposal.

It would be a mixed channel, drawing about half of its schedule from BBC One, 30 per cent from BBC Two, and the rest from BBC Three, BBC Four, CBBC and CBeebies. The nine-hour day would be flexible for coverage of live sports or other major events.

BBC HD will launch on Sky and Virgin as soon as it wins approval, and would be part of Freesat from its launch in 2008. The Freeview teaser would launch in mid-2008, and an internet stream would be added to iPlayer when it is technically possible.

It will run for around four hours daily at launch, building to nine hours by 2009. MPEG-4 would be used for the satellite and Freeview services, but MPEG-2 would be used for cable.

The service is expected to cost around £21.4million per year in 2012, if it's running on Freeview as well as satellite. But that assumes Ofcom will give it low-cost access to extra Freeview cpaacity - the BBC says it can't afford to bid at a commercial level.

The BBC's full HD proposal is available at the BBC Trust website. The first four-week consultation runs until June 19, with provisional conclusions in September and a final decision planned for November.

Ofcom is also running a Market Impact Assessment into the BBC HD service, which will be used for the Trust's final decision.