Channel 4 has pitched for the contract to sell Discovery Channel's ad space, following Discovery's disagreement with Sky Media, which currently sells its airtime.
Sky and Discovery are embroiled in High Court action but the contract is due to move at the end of this year.
Channel 4's attempts to increase its clout in the market have angered rivals because it is at the centre of government consultations that could result in greater levels of subsidy for Channel 4 via a partnership with BBC Worldwide.
Channel 4's rivals are also worried that Ofcom and the Government appear to have cleared obstacles to a merger with Five or a BBC Worldwide tie-up that could eventually result in a merger between Channel 4's sales operation and ids, the sales house that sells space across the UKTV and Virgin Media TV channels.
One senior TV sales executive said: "At a stroke, the Government has reset competition terms because poor old Channel 4 is in trouble."
Another said: "Channel 4 is using its status as a public-service broadcaster to secure control of private, commercial contracts."
Channel 4 declined to comment.