A pre-watershed ban on ads for junk food will prevent an estimated 20,000 cases of childhood obesity, the Government has said.

More detail on which food and drink products will be covered by the regulations are to published on Tuesday, as the Government confirmed the laying of secondary legislation for the ban.

The ban, which had been first put forward by Boris Johnson’s Conservative government in 2021, will come into force in October next year, after which television ads for junk food products will be allowed only after 9pm.

It will also include a ban on paid-for online ads for these products to reduce children’s exposure to foods high in fat, sugar or salt.

The Government said the ban was expected to remove 7.2 billion calories a year from UK children’s diets.

The Government’s impact assessment notes that “overall the studies do find a clear link between food advertising and calorie consumption”.

However, it also suggests that the measures could cut just 2.1 calories from children’s diets each day while costing advertisers £659 million in returns over 25 years.

It also warns that any calorie reductions achieved during childhood could be undone as individuals eat more during adulthood.

Health Secretary Wes Streeting said: “Obesity robs our kids of the best possible start in life, sets them up for a lifetime of health problems, and costs the NHS billions.

“This Government is taking action now to end the targeting of junk food ads at kids, across both TV and online.

“This is the first step to deliver a major shift in the focus of healthcare from sickness to prevention, and towards meeting our Government’s ambition to give every child a healthy, happy start to life.”

NHS data shows a trend of rising childhood obesity, with almost one in 10 reception-aged children (9.2%) now living with obesity and one in five by the age of five (23.7%) suffering tooth decay because of excess sugar consumption.

Professor Sir Stephen Powis, NHS national medical director, said: “NHS figures show that one in eight toddlers and primary school children are obese, and this is clearly a problem not only because we know that it could lead to young people having health issues later in life, but it’s also storing up problems for a future NHS which already spends billions dealing with the issue.

“We’ve always said the NHS can play its part in supporting people who are obese to reach a healthier weight, but we need to work with the rest of society to prevent people becoming overweight in the first place.

We are pleased the Government is to bring in legislation to restrict the advertising of unhealthy foods targeted at children and young people

“So we welcome this proposed legislation and look forward to working with the government and partners to help protect the good health of future generations.”

Children with obesity are said to be more likely to live with the condition as adults and to be at significantly greater risk of life-limiting illnesses.

Obesity is the second biggest preventable cause of cancer, according to health experts, costs the UK health service more than £11 billion each year, and is a major contributor to ill health that prevents people from participating fully in work.

David Fothergill, chairman of the Local Government Association’s Community Wellbeing Board, said: “We are pleased the Government is to bring in legislation to restrict the advertising of unhealthy foods targeted at children and young people.

“Childhood obesity is one of the biggest public health challenges we face. However, any efforts to tackle the causes of obesity need to be part of a whole systems approach.

“Greater powers for councils to tackle the clustering of takeaways and restricting junk food advertising near schools, alongside extra investment in council-run programmes such as those promoting physical activity and healthy weight can help play an essential part in helping to curb child obesity.”