It comes as speculation intensified in May that all the broadcasters and ad sales houses had been in continuous discussions about possible tie-ups in anticipation of the relaxation of the Contract Rights Renewal (CRR) mechanism.
The deal between Sky and Channel 4 is thought to have been agreed, though the latter has declined to comment on the matter.
The merger is part of a major cost saving strategy which is likely to lead to job cuts.
Many TV buyers have said that a consolidation of other sales houses is more than likely, but there is concern that it will lead to a stifling of competition to the detriment of clients.
One buyer warns however that the “danger is you will end up with only two sales houses competing against each other”.
A Sky spokesperson says it does not comment on speculation.