London authorities are being urged to tackle the “toxic school run”, as it emerges that children at 455 schools breathe air which breaches legal standards for pollution.

Campaigners are calling for the Mayor of London, Sadiq Khan, to prioritise the clean up around these schools, where raised levels of nitrogen dioxide can leave children struggling to breathe.

TfL, London’s transport body which is chaired by Mr Khan, is being questioned over its decision to put “dirty” diesel and diesel-hybrid buses on routes serving nine in 10 of these schools.

“It's the toxic school run: children are exposed to more than 60 percent of their air pollution intake at school and on their way there, which is why the government needs to tackle that first and prioritise that”, said Alastair Harper, a health campaigner at Unicef UK.

“If it’s 10 years until the air is cleaned up, then there's a whole generation of children forgotten.

“My own daughter was hospitalised due to air pollution. I’ve got skin in this game.”

As part of his bid to reduce air pollution, Mr Khan radically expand the Ultra-Low Emissions Zone, a scheme to charge drivers up to £24 a day to drive in London.