TV and radio companies wanting to merge could have to prove to the government that their move would not make the quality of programmes in the UK worse.
Ofcom replaced five broadcasting and communications watchdogs
If Culture Secretary Tessa Jowell thinks a proposed merger may not be in the public interest, she will ask new watchdog Ofcom to look into it.
The new rules could apply to any future mergers involving ITV or Channel 5.
But the £4bn merger of ITV companies Carlton and Granada was given the go-ahead before the new rules came in.
Ofcom took over as the UK's media regulator in December, with responsibility for TV, radio, telephone and hi-tech communications.
If Ms Jowell asks Ofcom to consider the impact of a merger, the body will look at three main questions and report back to the minister to make the final decision.
The tests also apply to cross-media mergers, between broadcasting companies and newspapers. They are:
There must be "a sufficient plurality of persons" in control of media outlets. This must apply to every different audience and every region in the UK.
There must be a wide range of high quality programmes available across the UK. These must appeal to "a wide variety of tastes and interests".
Those in charge must have "a genuine commitment" to things like accuracy, impartiality, fairness, taste and decency.
A merger is a coming together of two equally-sized companies. Ofcom also set out guidelines to determine whether newspaper mergers should go ahead.
They included tests of whether accuracy, free expression and a wide range of views would still be available if the merger took place.