The news will disappoint those advertisers who have lobbied against further consolidation in the light of the Government's decision to allow a merged ITV sales point.
However, the Office of Fair Trading may still investigate any merger that produces a TV sales house with a combined market share of more than 25 per cent of TV ad revenue.
"The ITC and Ofcom believe the robust provisions of the Competition Act and the increasing body of competition case law provide sufficient mechanisms to prevent distortions of competition on the joint selling of airtime," the regulator said.
All of the TV sales points have held informal discussions about possible sales mergers with their rivals to compete against a merged ITV controlling more than 50 per cent of the market.
Channel 4 has the biggest share of the TV ad market after ITV, with around 20 per cent of revenue. The multi-channel operators, including Sky Media, take 18 per cent, while five has 8 per cent.
In a statement, Channel 4 welcomed Ofcom's decision. A spokesman said: "It made no sense to keep these rules in place after the Carlton and Granada merger had been approved. We believe competition law will be an adequate defence against market dominance. However, we have no plans at present to change the basis on which we sell our airtime."
The ITC and Ofcom have also, in the light of the ITV merger, removed restrictions forbidding the network to negotiate share-of-TV deals. Conditional selling, where broadcasters force advertisers to buy airtime they may not want, remains outlawed.
The rules have changed with immediate effect.