A crackdown on advertising for unhealthy food to help tackle childhood obesity officially begins today.

The ban prevents ads for “less healthy” food and drink that is high in fat, salt and sugar (HFSS) appearing on television between 5.30am and 9pm, or online at any time.

A voluntary ban by advertisers began on October 1 but from today they must now comply with the new rules or risk action by the Advertising Standards Authority (ASA).

The ban applies to products that fall within 13 categories considered to play the most significant role in childhood obesity, including soft drinks, chocolates and sweets, pizzas and ice creams, but also breakfast cereals and porridges, sweetened bread products, and main meals and sandwiches.

Products that fall into these categories are then also assessed as to whether they are “less healthy” based on a scoring tool that considers their nutrient levels and whether products are high in saturated fat, salt, or sugar.

Only products that meet both of the two criteria are included in the restrictions. Companies can still advertise healthier versions of products included in the ban.

Latest figures suggest one in 10 reception-aged children are obese, while one in five children have tooth decay by the age of five.

It is estimated obesity costs the NHS more than £11 billion every year.

The Government estimates the ad ban will prevent around 20,000 cases of childhood obesity.

Katherine Brown, professor of behaviour change in health at the University of Hertfordshire, said: “A watershed ban on television and online advertising is long overdue and a move in the right direction.

“Children are highly susceptible to aggressive marketing of unhealthy foods and exposure to them puts them at greater risk of developing obesity and associated chronic diseases.”