The planned merger of the big ITV companies Carlton and Granada has been referred to the Competition Commission.
Charles Allen's plans to join forces with Carlton could be blocked
Trade & Industry Secretary Patricia Hewitt said she wanted the Commission to look at the plan mainly because of concern about the sale of advertising.
She said she had made the decision on the advice of the Director General of Fair Trading .
"The merger would greatly increase concentration in TV advertising, leaving one firm with more than half of national TV advertising revenue," Ms Hewitt said in a statement.
The heads of the two TV companies denied that, by joining forces, they would reduce competition.
"This merger will not affect the competition for viewers," said Michael Green, chairman of Carlton.
"ITV is already one network and advertisers follow viewers and viewers follow programmes."
Granada's chairman Charles Allen added that a united ITV could "grow in the future and deliver for our viewers, advertisers and shareholders".
Both companies have been hard hit by the downturn in advertising.
They also made big losses from their disastrous investment in the failed pay-TV channel ITV Digital.