Project Kangaroo, the online collaboration between BBC, ITV and Channel 4, would restrict competition in the UK, according to the Competition Commission.
The provisional findings from the Commission's investigation into the project suggest that video on demand (VOD) services would suffer against the might of the three broadcasting giants.
There would be a 'substantial lessening' of competition in UK VOD, according to the commission.
The concept of Kangaroo is a sprawling video on demand service allowing viewers across the country to access programmes when they want them.
Although BBC, through the iPlayer, Channel 4's 4OD and ITV's website all offer differing levels of service – a collaboration would create a central platform that could sweep rivals aside.
Competition Commission chairman Peter Freeman said: "Video on Demand is a relatively new and rapidly-expanding medium and [Kangaroo] clearly has much to offer.
"However, we are concerned that a loss of rivalry between BBC Worldwide, ITV and Channel 4, who are normally regarded as close competitors, could restrict existing and future competition for VOD.
"Whatever benefits viewers would gain from this rivalry would clearly be lost."
Project Kangaroo was not for catch-up TV which would have remained on the individual broadcasters' services, but would instead have offered an archive of material.
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