Billboards advertising junk food offered by vendors like McDonald's might be banned from London's Underground rail and bus network under new plans announced by the city's mayor as part of his efforts to tackle rising levels of childhood obesity.

Sadiq Khan, elected mayor of London in 2016, said he wants to reduce the influence and pressure put on children and families to make unhealthy choices. Nearly 40 percent of 10-and-11-year-olds in the capital are overweight or obese, one of the highest rates in Europe, the mayor's office said.

A large percentage of the advertising that would be affected comes from "a handful" of major companies and brands, said a spokesman for Transport for London, a governmental body responsible for the city's transportation system. A spokeswoman for McDonald's, whose ads are a regular fixture at Underground stations, said the firm shares the mayor's ambition to reduce childhood obesity.

"We take our responsibilities extremely seriously and have always complied with and exceeded the stringent marketing requirements placed on us — some of the strictest in the world," she added.

Stephen Woodford, chief executive of the Advertising Association, a lobby group for the U.K. ad industry, said research has shown an advertising ban would have little impact on the wider societal issues that drive obesity.

The U.K. already bans the advertising of high-fat, high-sugar and high-salt food or drink products in media where under 16-year-olds make up more than 25 percent of the audience, including TV, online, social media and in the street or on public ­transport.

Food and drink advertising contributed about 20 million pounds ($27 million) to Transport for London's revenue during the 2016-17 financial year, with about two-thirds of that coming from "high fat, salt and sugar" food and drink, the spokesman added. That's just a fraction of the 5.4 billion pounds Transport for London earned in gross income.