Following a ‘beauty contest' tender process, Channel 4 will be able to broadcast an HD simulcast of its main channel on DTT.
At the same time Channel 3 licensees (ITV plc, Channel TV, stv and UTV) will transmit their peaktime schedule and, outside peak, offer on-demand services and sub-let space to other broadcasters.
C4 and ITV join the BBC in launching HD services on DTT, expected to start with the Granada region from late autumn 2009.
Nationwide coverage will be available by the end of digital switchover in 2012.
Ofcom chief executive Ed Richards said: "This is a significant step forward in the range of choice available in the UK. It will enable viewers to watch some of the very best programmes free-to-air in high definition through their television aerials."
To access the HD programmes and new services on DTT, viewers will need an HD Ready TV set and an HD set-top box.
Ofcom believes that a fourth HD service could be launched on DTT by 2010.
Channel 4's pitch was done in conjunction with Welsh-language broadcaster S4C. Both stated that their service will also include:
a strong film component (4HD expects to broadcast over 150 hours of films in HD during the first twelve months, with the majority shown in peak-time);
drama, comedy, science programmes and documentaries made in the UK in HD and programmes imported from the US such as Desperate Housewives;
viewers of S4C HD will be able to watch Welsh-language programmes such as children's programmes and European Cup Rugby games in HD.
In addition, the joint Channel 4 / S4C proposal may offer in future an on demand service or sub-let during overnight hours.
The new ITV HD service will also include:
sport programmes, such as the FA Cup and the 2010 Football World Cup, in HD;
newly-commissioned drama series such as Miss Marple in HD;
programmes about the UK's nations and regions in place of the early evening and late night regional news services.
The BBC Trust will decide on the BBC service to be carried on the third of the three slots currently available.