The BBC Trust's researchers have urged the BBC to launch its high definition channel as soon as possible on Sky, Virgin and Freesat.

But any plans for an overnight or full-time service on Freeview should wait until the BBC can judge when new transmission technology will be available.

The BBC wants to launch a nine-hour daily hi-def channel on satellite and cable, with a four-hour overnight service on Freeview for viewers with HD PVRs.

The BBC Trust's research unit has been studying the proposal to help the Trustees make a decision later this month.

The research unit's report said the current proposals will "deliver a medium to high level of public value" by helping to promote the development of HD in the UK, and improve the quality of the BBC's services.

It added that the nine-hour satellite and cable service should consider simulcasting other BBC channels in prime-time, as well as BBC One.

However, it said the launch of any hi-def on Freeview would be complicated by Ofcom's proposals to reorganise the Freeview multiplexes to make more efficient use of them.

The BBC wants to use spare capacity among its own channels from the middle of next year, then get a new Freeview multiplex for hi-def after the digital switch completes in 2012.

Ofcom's plans haven't been published in full, but rely on the use of both MPEG-4 compression and the forthcoming DVB-T2  technology for Freeview, which won't be on the market until late 2009 at the earliest.

So the Trust's researchers have recommended there's no decision about a Freeview service until early 2008, when Ofcom's plans and more details of DVB-T2's launch date will be available.

Ofcom yesterday gaves its approval to the BBC's HD plan, but also asked for a delay before launching a channel on Freeview.