Channel 4's Ugly Betty is already shown in high definition in the US
Channel 4 is to be broadcast in high definition later this year, the first time a UK terrestrial broadcaster has simulcast its output in this way.
Viewers with a compatible TV and Sky HD box will be able to watch Channel 4 HD without paying an extra subscription.
However, only programmes recorded using high-definition techniques will be screened in HD, a format which promises better-quality pictures and sound.
Other shows will be seen in standard definition, a Channel 4 spokesman said.
He added that the proportion of programmes shown in HD would increase over time.
The BBC has offered a trial HD service since last year, but this free service carries a customised selection of programmes, rather than simulcasting an existing service.
Channel 4 said it "anticipated" that its HD service would launch in December.
Expanded BBC service
The announcement came a day after the BBC Trust - the body which must be consulted before any new BBC channel can begin - indicated it was in favour of a permanent BBC HD service.
BBC executives have proposed a daily nine-hour schedule - from 1500 to 0000 - on satellite, cable and Freeview.
Michael Palin's latest travel series is being screened on BBC HD
However, this would only be possible on Freeview once analogue signals had been switched off across the UK in 2012, owing to the lack of space on the airwaves.
An interim measure - if approved - would see BBC HD on Freeview between 0200 and 0600 daily.
But BBC Four, BBC Parliament and three BBCi services would have to be taken off air during those times to provide the necessary bandwidth.
The trust set down several conditions for the overall HD service, saying it was important to broadcast material from a variety of channels during peak-time evening viewing, rather than simply simulcasting BBC One.
However, it concluded overall that the service "would deliver a medium to high level of public value".
A further set of provisional conclusions on BBC HD will be published by the trust next Tuesday, and the public will then be able to submit their views during a month-long consultation process.
A final announcement on the service will then be published in November.