The BBC Trust has approved the corporation's plans to launch a high definition television channel, saying it would be of "significant public value".
But it also asked viewers to voice an opinion on whether they want the new technology to be made available immediately on Freeview, which would involve buying a new set-top box now, and another in one to four years' time, or whether they are prepared to wait. A public consultation on the subject is being held until 23 October.
Trustees also revealed the cost of the channel – a total of £21.5m by 2012.
High definition, which offers a far superior picture quality, enhancing nature programmes, dramas and coverage of sporting and music events, can be made available almost immediately on satellite and cable television – where a trial of the service is already running. The problem lies with the digital terrestrial television platform, Freeview. At present, no Freeview box is equipped to carry high definition and in addition there is not sufficient digital spectrum available to run the nine-hour HD service which is proposed for satellite and cable.
One option is to run the HD channel on Freeview between 2am and 6am, which would mean taking BBC Four and BBC Parliament off air during these hours. Viewers would then be obliged to buy a new HD-ready set-top box, preferably one with a video recorder included, so that they could record the early-morning programmes to watch later.
Within one to four years, however, this set-top box could become obsolete and viewers would have to purchase a second set-top box, which would enable them to watch the full nine-hour service on Freeview.
Ofcom, the media regulator, is carrying out a review of spectrum and the BBC Trust wants to postpone a decision about Freeview and HD until it has reported early next year.
Diane Coyle, a BBC trustee, said: "Our view is that these proposals would deliver significant public value and create very little adverse market impact." She added: "We are particularly interested in the public's view about the options for an HD channel on Freeview. Would licence fee payers prefer the BBC to launch a four-hour service immediately, before a nine-hour service is possible because of spectrum capacity, even if that means having to buy two set-top boxes within a very short space of time?"
Channel 4 announced last week that it will launch a high definition service on Sky later this year, which will be the same as the main channel, but screened in HD where possible.
The BBC already shoots many of its programmes in high definition, including Planet Earth and Bleak House, for sale to the international market.