The British government said Thursday it will bring in an evening watershed for junk food ads in a move to tackle the nation's obesity problem.

The state-funded National Health Service (NHS) estimates that some 10 percent of four- and five-year-olds are obese, a figure that climbs to 20.2 percent for those aged 10 and 11, or one in five.

It adds that one in four adults are obese, with cheap, high-calorie foods blamed in part.

From the end of next year, television and on-demand channels will not be allowed to advertise food high in fat, sugar and salt before 9:00 pm.

Public health minister Jo Churchill said the new policy aimed to protect children from "unhealthy advertising" and "wipe billions off the national calorie count".

It cited analysis of advertising from 2019 that found that almost 60 percent of ads on the main commercial TV channels between 6:00 pm and 9:00 pm were for foods high in fat, sugar and salt. 

There will be some exceptions for foods that fall in these categories but are relatively unprocessed and healthy -- such as marmite or honey -- and for small and medium-sized food businesses.

The new rules do not ban all junk food promotion online, despite calls for this from campaigners including celebrity chef Jamie Oliver.

Junk food companies will be able to put out content that is not paid-for, such as on their brands' social media pages and blogs.

Prime Minister Boris Johnson's government is seeking to "level up" health inequalities in more disadvantaged regions of the country. 

The likelihood that overweight people will suffer worse outcomes from Covid-19 infection has focused attention on the problem.

Johnson himself has struggled with his weight, blaming it for contributing to his battle to shrug off Covid-19 last year, which saw him spend several days in intensive care. 

He has since boasted of giving up "late-night cheese" and been spotted running with a personal trainer in a park near his Downing Street office.

"I am fitter than a butcher's dog, thanks basically to losing weight," he said in September last year.