Britain's first full-time Freeview HD channels could launch as early as 2009, with up to four on air by 2012.

Media regulator Ofcom has greenlit its own proposals to upgrade one of the six Freeview multiplexes to highly efficient next-generation transmission technology.

This could allow high definition broadcasting or dozens of extra standard definition channels, but viewers will need a new set-top box or a plug-in module for their IDTV.

But the plan doesn't look beyond digital switchover in 2012 to suggest how Freeview could continue to develop in the long term.

The new services would use Multiplex B - currently operated by the BBC - to transmit channels using MPEG-4 compression and the cutting-edge DVB-T2 transmission standard. Together these can effectively double the capacity of this multiplex.

The current channels on Mux B would be moved to either the BBC's Mux 1, Mux 2 (owned by ITV and Channel 4) or Mux A (owned by SDN) - here's an illustration.

The upgraded Mux B would start operating in 2009, when the Granada, Wales and Westcountry regions switch of their analogue TV.  Other regions would get the extra channels as they switch to digital, except for Border, which will switch later this year and would have to be upgraded again before 2012.

Mux B would be expected to carry three high-efficiency channels from 2009, but it's expected that continuing improvements to the new technology would allow four or five channels by 2012.

On of the channels would be controlled by the BBC Trust and is expected to be used for the BBC HD channel which is already available on satellite and Virgin Media.

The others would be auctioned to ITV, Channel 4 and Five - although the broadcasters have already agreed that ITV and Channel 4 will launch hi-def channels first, with Five joining when the extra capacity arrives.

But Ofcom will not insist the extra capacity is used for HD - the channels could propose to launch a wealth of new SD channels instead.

Digital TV experts at the Digital TV Group have asked Ofcom to create a plan for Freeview beyond Digital Switchover in 2012, but the latest plan doesn't run past 2014, when the new licenses for Mux B will run out.