Planet Earth has been screened on the BBC's HD trial service

The proposed upgrade could see HD TV available on Freeview as early as 2009.

However, viewers who wish to watch HD will need to buy a new set-top box, as well as an HD-compatible television.

The industry watchdog's announcement followed Tuesday's news that the four terrestrial broadcasters have agreed to deliver HD channels on Freeview.

In a joint statement, the BBC, ITV, Channel 4 and Five said they had signed a "non-binding memorandum of understanding" pledging to launch HD services between 2009 and 2012.

Problematic

Ofcom said the DTT upgrade could be completed by late 2009, in time for the scheduled digital switchover in the Granada television region.

The new services would then be available as terrestrial services are switched off across the rest of the UK.

Viewers wishing to access HD need a compatible television

The launch of HD on Freeview has been problematic due to the limited capacity currently available for DTT services.

Through a combination of new technology and efficiency improvements, however, Ofcom believes the capacity "could, in time, more than double".

HDTV is a new format that enables customers to access sharper pictures, improved sound and widescreen images.

The BBC is currently broadcasting a trial version on satellite and cable, with Channel 4 set to launch its own service next month.