Bearing the working title Kangaroo, the new service will complement the broadcasters' various online media initiatives: BBC iPlayer; ITV.com; and 4OD.
It will house more than 10,000 hours of content with a focus on catch-up TV and archive programmes.
Lesley Mackenzie, the launch chief executive, is a former director of channels and operations at BSkyB and most recently launched TataSky, a satellite TV joint venture between Sky and Tata, in India.
The three are equal partners and will share revenue, regardless of which one of them owns the content that generated it.
Revenue will come from advertising and from consumers paying to "rent" or permanently download some content, while other content will be free.
The broadcasters are clearly in a hurry to launch the service, which they noted needs to be formally approved by the BBC Trust and each of their boards. This could put to the test theories that Sky's 17.9% stake in ITV could allow it to block the company's investment plans.
The trio say Kangaroo will help them better compete in the rapidly expanding online content space, pool their knowledge, and share the cost of developing new technology.
They will encourage third-party content owners to join the service, and plan to make it available on other platforms after the internet.
John Smith, CEO of BBC Worldwide, said: "This is a historic partnership between the UK's largest broadcasters... By combining our joint resources we're really taking control of our destiny in a market that's moving at a fast pace."
Michael Grade, executive chairman of ITV, said: "This key strategic move is complementary to our investment in ITV.com and further strengthens ITV's customer proposition online. The joint service has the potential to become an important shop window for UK broadcaster content and a great destination for viewers."
Andy Duncan, chief executive of Channel 4, said: "TV on the internet is a rapidly growing medium as proven by the incredible success that Channel 4 has enjoyed with its 4oD service. We want to continue to build and innovate in this area, giving viewers ultimate control over what they watch and when they watch it, and believe that partnering and sharing expertise is the best way of doing this.".jpg)
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