Campaigners are seeking to take the TV regulator Ofcom to court over its refusal to consider a pre-watershed ban on junk food advertising.
The consultation is due to end next month
The National Heart Forum said it is preparing an application for a judicial review over the consultation the regulator is currently carrying out.
Ofcom is consulting on restricting junk food ads, but has already ruled out banning their broadcast before 9pm.
The heart charity said it was unlawful and unfair to exclude the possibility.
Deputy chief executive Jane Landon said: "We are dismayed that Ofcom has weighed industry profits against children's health, compromised this important consultation and forced us to take this unprecedented step of seeking a fair consultation through the courts.
"What we are asking Ofcom to do is to consider the 9pm watershed option fairly and equally alongside the far more limited options it has consulted on.
"This will ensure that parents know the full range of possibilities and Ofcom can hear their views.
"We are not asking for much, but we believe a fair and open consultation is essential to ensure that Ofcom reaches the right decision."
The charity is being supported by a range of medical and patient bodies including the British Medical Association, the Royal Society for Public Health and Diabetes UK.
Dr Vivienne Nathanson, of the BMA said: "The BMA has been calling for the regulation of advertising of unhealthy foods to children for a year. It is time for an open consultation offering firm action not just half-measures."
When Ofcom published its consultation in March the proposals were heavily criticised for being too weak.
Under the plans, celebrities and characters from films or TV programmes would not be allowed to take part in any food or drink commercial targeted at the under 10s.
In 2004, a government white paper on public health said children should not be encouraged to eat too many foods high in fat, salt and sugar. At the time, government sources suggested that this could mean an ad ban.
Any changes to food promotion must be introduced by next year, the white paper said.
When it launched the consultation, Ofcom argued a pre-watershed ban would cost broadcasters up to £240m every year in lost revenue and would be disproportionate.
Ofcom research indicates TV adverts have a "modest direct effect" on children's food choice.
But the combination of exercise, family eating patterns and school policy plays a much larger role in childhood obesity, it said.
A spokesman for Ofcom said while an ad ban was not a specific proposal, the regulator would still "welcome public and detailed views on this point, as well as fresh and substantive evidence, if it exists".
The consultation is due to end next month.