Children should be "bussed in" from the countryside to breathe in London's air Boris Johnson claimed this week, as new figures revealed toxic levels of pollution in the capital.

The mayor of London claimed the city's air is now "alpine" in its cleanliness.

"The air quality in London, you can go outside and breathe in great gulfs of virtually alpine air," he told the conservative think tank the Capital City Foundation earlier this week.

"There was a recent bad air day - this is absolutely true - where the air quality in Norfolk was inferior to the air quality in London.

"The day is not far my friends when the children of Norfolk will be bussed in order to inhale our superior air."

Scientists have linked air pollution to respiratory problems and the stunted lung growth of children in London. Even the mayor's own officials estimate that air pollution currently contributes to the deaths of around 4,000 Londoners every year.

The mayor's opponents today criticised him for "making light" of the issue.

"Knowing that children can be profoundly and permanently affected by diesel fumes, making light of the problem is indefensible," Green Party Lodnon Assembly Member Jenny Jones told Politics.co.uk.

Johnson's comments come as new figures released by the Department for Environment reveal London's roads are the most polluted in the country.

The findings released under freedom of information, suggest that some of London's most polluted stretches of road could be made even worse under plans being pursued by Johnson and the government.

Map of London's most polluted roads v/ @BorisWatch Boris's plans would make these even worse http://t.co/nHjFHbO3o3 pic.twitter.com/LrzhB2PO03 — Adam Bienkov (@AdamBienkov) February 20, 2015

Levels of nitrogen dioxide, which is linked to premature deaths and respiratory problems, were found at extremely high levels on roads connecting to the site of Johnson's planned Silvertown river crossing between East and SE London.

Johnson plans to build a new tunnel feeding into the existing crossing at Blackwall as well as a further two crossings further east.

His transport deputy Isabel Dedring recently admitted that these new river crossings could double levels of traffic on local roads.

"People are going to say 'I like that idea of a bridge but not when I discover that it is going to lead to a doubling of traffic on my road'," she told MPs.

"And that's inevitable with these kinds of projects."

Targets to tackle dangerous levels of pollution in London were last year pushed back beyond 2030. Ministers had originally agreed to solve the problem by 2010.

Continued failure to reduce dangerous pollution levels could see the UK hit with huge EU fines.