Although Granada is not expected to table an immediate hostile bid for either company, it is thought to believe that Carlton and United have put themselves in play by agreeing a no-premium deal.
European groups, including Vivendi of France and Mediaset, the Italian operator controlled by Silvio Berlusconi, may also be planning separate bids for United or Carlton.
The logic behind a hostile bid comes from the fact that the proposed pounds 8bn merger of Carlton and Granada undervalues both companies substantially. Analysts note that a hostile bid for either company would probably come in at pounds 5bn or more, a 25 per cent premium to the companies' respective market values.
Although shares in both companies jumped when the merger was unveiled, waves of selling to lock in profits saw the stock of both close only modestly higher. Commenting on the market underperformance of Carlton and United, traders said many investors were using the opportunity to cut losses and move on amid fears that the regulatory authorities might modify or even scupper the deal.
Carlton closed on Friday only 4 per cent up at 576.5p - well below its 12-month high of 656.5p. United fared even less well, ending up 3.2 per cent at 769p, well below its all-time high of 902p reached last year.
Granada is also considering whether the bid, which amounts to a reverse takeover of Carlton by United, also signifies a change in control of Carlton. This could allow Granada to buy out Carlton's 50 per cent interest in ONdigital, the money-losing digital terrestrial television broadcaster.
Granada and BSkyB are expected to lodge complaints with the Office of Fair Trading, which is reviewing voluntary competition undertakings that limit any one ITV group to 25 per cent of total television advertising. The Carlton/United merger would give the new group 36 per cent of commercial television advertising sales. The review may be completed next month.
As Britain's biggest commercial television player, Granada had been expected to play a lead role in reshaping the ITV system. Earlier this year it snapped up the 18.6 per cent interest in Scottish Media held by Mirror Group, amid speculation that a foreign buyer was attracted by the stake.