Junk food adverts will be banned in all tube stations and bus stops across London from February under new plans to curb the “ticking time bomb” of child obesity.

The capital has one of the highest child overweight and obesity rates in Europe, with almost 40 per cent of 10-11-year-olds deemed an unhealthy weight.

Obesity cost the NHS more than £6.1bn last year and it is also considered a driver of health inequalities, as children from more deprived areas are disproportionately affected. A child in Barking and Dagenham is almost twice as likely to be overweight as one from Richmond.

The high-profile move is likely to ramp up pressure on Theresa May, nearly two years after she watered down plans to restrict adverts and promotions in supermarkets.

Despite major commitments from the government to tackle the issue, campaigners have warned that more needs to be done to stop the problem spiralling out of control.

Mr Khan said: “Child obesity is putting the lives of young Londoners at risk and placing huge pressure on our already strained health service.

“It is absolutely imperative that we take tough action against this ticking time bomb now, and reducing exposure to junk food advertising has a role to play in this – not just for children, but parents, families and carers who buy food and prepare meals.”

He added: “It’s completely unacceptable that in a city as prosperous as London, where you live and the amount you earn can have a massive impact on whether you have access to healthy, nutritious food.”

Adverts for unsalted nuts, raisins and sugar-free drinks will still be allowed but products deemed “less healthy” by Public Health England will be banned, such as chocolate bars, burgers and fizzy drinks, under the reforms.

From February 25, the restrictions will apply to advertisements on all modes of transport controlled by TfL, including the underground, overground, London buses, trams and river services.

It follows a similar ban in Amsterdam last year, where curbs were brought in to limit the amount of exposure children have to junk food ads.

Young people who are bombarded by adverts every day were more than twice as likely to be obese, according to a report by Cancer Research earlier this year.

The same study found 87 per cent of young people found adverts for high fat, salt and sugar products appealing, with three-quarters tempted to eat a product after seeing such an advert.

Chef Jamie Oliver, a campaigner against child obesity, said: “This is an amazing move from the mayor and TfL, and they’ve got overwhelming support from Londoners who’ve said loud and clear they want a transport system with healthier ads and messages.”

Hugh Fearnley-Whittingstall, chef and campaigner said: “When it comes to tackling childhood obesity, we need to pull all the levers possible, which is why I’m delighted to hear that not only are the mayor of London and TFL removing junk food ads on their network, but they’re actually going to promote vegetables instead.

“This is a real victory for veg - let’s hope TFL paves the way for others to follow.”

Health experts also offered their backing for the plan, with chief medical officer Dame Sally Davies describing it as an “important step” to winning the war against child obesity.

“We know advertising directly influences children’s choices so this very welcome announcement has come at a crucial time,” said Professor Russell Viner, President of the Royal College of Paediatrics and Child Health.

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Food and drink brands will be able to appeal to advertise less healthy products if they can provide evidence their snacks do not contribute to poor diets among young people.

Stephen Woodford, of the Advertising Association, the industry body, said: “There is no clear evidence that a ban on high fat, salt and sugar advertising on the TfL out-of-home estate will have a positive effect on reducing childhood obesity rates in London.

“We all want to see rates of childhood obesity dropping but believe there are far better ways to achieve this goal.