Junk food advertising on television should be banned before the 9pm watershed in an effort to tackle soaring obesity rates among young people, the leader of Britain's children's doctors has urged.

A clampdown on advertising foods high in salt, sugar or fat would help protect children from unwelcome and unhealthy "commercial exploitation", said Dr Hilary Cass, president of the Royal College of Paediatrics and Child Health, which represents 11,500 children's health specialists across the UK.

Existing regulations are too weak and need to be overhauled, she said. "Although they are trying to avoid junk food advertising around specific children's programmes, you've still got it around soaps and other programmes that children watch. So the only realistic way to do it is to have no junk food advertising before the watershed in any programmes at all."

"When children see the adverts they start nagging their parents to get them a McDonald's or whatever. They see something at 6pm on the telly and want a McDonald's that night. It's a similar thing to having sweets at the checkout – get to them then", added Cass, a senior paediatrician at St Thomas's hospital in London.

The coalition should also impose immediate extra taxes on highly-sugared soft drinks and examine the viability of "fat taxes" to reduce consumption of unhealthy foods, she added.

The Food and Drink Federation, which represents big food manufacturers, declined to comment. But Cass's plea was rejected by the Advertising Association, the industry's trade body.

"This call for a watershed ignores the academic evidence and risks overlooking the real causes of childhood obesity", said Sue Eustace, its director of public affairs. "Advertising in the UK has an exemplary record in complying with one of the strictest regulatory regimes in Europe, and is already playing its part with constructive changes to the volume, visibility and content of food ads."