Video sharing site YouTube has taken online viewing to the masses
Some 43% of Britons who watch video from the internet or on a mobile device at least once a week said they watched less normal TV as a result.
And online and mobile viewing is rising - three quarters of users said they now watched more than they did a year ago.
But online video viewers are still in the minority, with just 9% of the population saying they do it regularly.
Another 13% said they watched occasionally, while a further 10% said they expected to start in the coming year.
The success of sites like YouTube over the past year has helped open the door for those who want easy ways to find, watch and share videos over the internet.
The UK is not yet as advanced as the US, where hit TV shows are routinely available from networks' websites and services like iTunes.
But it is catching up, with the BBC, ITV and Channel 4 all planning to offer most of their shows on demand on the internet from the end of this year or the start of 2007.
In the survey, one in five people who watched online or mobile video at least once a week said they watched a lot less TV as a result. Another 23% said they watched a bit less, while just over half said their TV viewing was unchanged. Some 3% said online video inspired them to watch more TV.
Online and mobile video is far more popular among the young, with 28% of those aged 16-24 saying they watched more than once each week.
An average of 10% aged 25-44 were net video regulars, with that figure falling to just 4% of over-45s.
Earlier this year, media regulator Ofcom said the number of 16 to 24-year-olds watching TV in an average day had dropped 2.9% between 2003 and 2005.
Comedian Ricky Gervais, whose audio and video podcasts have become hits on the web, said amateur video would never replace TV - but broadcasters would harness the power of the internet.
"You can't knock up an episode of The Sopranos or 24 on a little handheld digital camera," he told the BBC News website.
"I don't think you'll ever be able to sidestep TV or DVD. But TV companies will embrace it."
"I'm sure when the BBC first launched, they were going: 'Ah, not many people have got tellies. Who's watching this?'
"So it's good to get your act together. And then people catch up with the know-how and the means to watch it."
The first award ceremony for web-only video, the Vloggies, was held in San Francisco at the start of November.
Alive in Baghdad, a site featuring videos of real Iraqis telling their own stories, won the top award.
Panellists were recruited from across the country and the results have been weighted to the profile of all adults. ICM is a member of the British Polling Council and abides by its rules.