According to the proposals, an HD service could start as early as 2009, but would require viewers to ditch their current set-top boxes for ones that could decode the new signals.

The watchdog announced that the four main terrestrial broadcasters (BBC, ITV, Channel 4 and five) had signed a "non-binding memorandum of understanding" pledging to launch HD services between two and five years time.

The proposals outlined how the service could be upgraded. It said the digital switchover would allow a change in broadcast technology that would increase the capacity of DTT by 20 per cent (which would mean an extra channel per mux). Also, improving efficiency in current broadcast technologies could squeeze out some extra bandwidth.