The Competition Commission has blocked the online joint TV venture between BBC Worldwide, ITV and Channel 4.
The Commission said, the Kangaroo Project as it is known,would have a detrimental impact on competition in the video-on-demand (VOD) market. VOD makes programmes available to viewers at any time over the internet.
Evidence it collected showed viewers value UK-made programmes above other forms of content, said the commission.
The problem, it said, is that BBC Worldwide, ITV, and Channel 4 control most of these programmes, putting them in a "very strong position as wholesalers of TV content to restrict competition from other current and future providers of VOD."
The commission said that none of the possible remedies that might have diluted the monopoly and maintained competition in the market went far enough.
ITV chief executive Michael Grade said the company was "surprised" by the decision."We believed that the Kangaroo joint venture, competing in a crowded online world against dominant global brands, was an attractive UK consumer proposition," he said.
"However, in the two years since the idea for Kangaroo was born, the success of ITV.com has proved that our UK content is attractive enough to stand on its own and we remain focussed on our online growth."
He said ITV would realise further news on its online plans in its full-year results on March 4.