The proposal is being driven by Granada's Charles Allen and Carlton's Michael Green. It would enable the companies to generate £20 million of savings but will raise fears of ITV's monopoly position.
Observers have suggested Granada, which is the dominant player in the merger, is simply trying its luck. But the news has concerned agencies and advertisers.
Jim Marshall, the chairman of the IPA media policy group, said: "The IPA is surprised and disappointed they've taken this view. There has to be competition. If we can't be assured of a competitive market, we'll oppose the merger."
Ian Twinn, the director of public affairs at ISBA, added that the idea of a single sales house was a "non-starter".
Last month, it emerged that Martin Bowley, Carlton Sales' chief executive, was planning a management buyout of his sales division if regulators demand two sales points.
But Marshall added that even in the event of two independent ITV sales houses there were still reservations. "We would want their working practices carefully policed by advertisers and agencies and if there were any transgression they would be reported to Ofcom," he said.
Nicola Howson, the director of corporate affairs at ITV, said the issue was a matter for Carlton's and Granada's sales houses but no-one at either operation would comment.
The submission is expected to arrive at the OFT at the end of this month.
It will provide a report to the culture secretary, Tessa Jowell, and it is almost guaranteed she will then refer the matter to the Competition Commission.