Carlton Communications and Granada control more than 50% of the UK TV advertising market, which is considered anti-competitive on the grounds that it is a monopoly.
Carlton and Granada's merger plans are widely expected to be referred to the Competition Commission, which is expected to object to the merger of their two sales houses on anti-competitive grounds.
If, as the broadcasters have asked, their share of the ad market is viewed as just part of the market as a whole, their combined market share is just 18%.
The advertising industry is largely against the merger of Carlton and Granada's sales houses because it believes that it will push up the price of advertising.
Channel 4 and Five have said that they will merge their sales houses if the ITV merger goes ahead. There is also talk that the two are trying to woo BSkyB into joining any sales merger.
Since they announced their £2.6bn merger last month, Carlton and Granada have said that, overall, it will benefit advertisers because it will deliver larger audiences.
This morning, shares in Carlton and Granada were up following positive full-year results from both companies earlier in the week.
Carlton was up 6% or 9p at 11am to 159p, Granada rose 5.06% or 4.5p to 93.5p.
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