A ban on adverts for junk food during television programmes aimed at children under 16 has come into force.
Regulator Ofcom has outlawed adverts for foods high in fat, salt and sugar in an effort to tackle rising childhood obesity levels.
But broadcasters say the quality of children's programmes will be hit by the loss of an estimated £39m in advertising revenue.
Health campaigners had called for a complete ban before the 9pm watershed.
The move is the latest stage in a phased crackdown on advertising during programmes aimed at or appealing to children.
In April 2007, junk food ads were banned during programmes made to appeal to seven to nine-year-olds.
And by December this year, dedicated children's channels will have to phase them out altogether.
Children's Secretary Ed Balls has said that UK children see some 10,000 television adverts a year and recognise 400 brands by the age of 10.
Terrestrial broadcasters have predicted their advertising revenue will fall by 1% after the ban.
Child-orientated satellite channels expect a 9% drop, while commercial channels aimed entirely at children fear a 15% fall.
Ofcom's rules impose curbs on adverts during shows where child viewers make up a high percentage of the total audience.
But in November, consumer group Which? claimed the restrictions were insufficient because they did not cover family programmes which appealed to both children and adults.
Among these were high-profile shows such as The X Factor, Ant and Dec's Saturday Night Takeaway, New You've Been Framed and Coronation Street.
Richard Watts from the Children's Food Campaign told BBC Radio 4's Today programme that 18 out of the top 20 shows watched by children were not covered by the new ban.
"The rules are fantastically complicated and opaque for parents," Mr Watts said in endorsing a complete ban before the 9pm watershed.
He accused Ofcom of balancing the protection of children's health alongside the "financial health" of broadcasters.
In addition to scheduling restrictions, Ofcom plans to ban the use of celebrities and characters, such as cartoon heroes, to advertise unhealthy food.
Free gifts and health or nutrition claims will also be banned.
A Food Standards Agency ratings system is used to assess which foods are deemed to be junk products.