Rosamund Kissi-Debrah told Sky News that "yes, (ULEZ) is painful, it's costly", but added: "Stunted lung growth will cost us on the NHS - that's your taxes and my taxes."

Between eight and 12 children with asthma continue to die in London every year, Ms Kissi-Debrah said, while across the four nations of the UK it is between 22 and 24.

During the first COVID lockdown, between late March 2020 and early July, it dropped to zero, she said.

"Five weeks after we reopened, a young boy died in London."

Ms Kissi-Debrah added: "These are preventable deaths. This is a public health crisis."

During an inquest in late 2020, a coroner ruled that the death of her daughter, Ella, could be attributed to air pollution.

The nine-year-old lived just 25 metres from the South Circular Road in Lewisham, southeast London - one of the capital's busiest roads.

Ms Kissi-Debrah said she waited up until midnight so she was awake when the ULEZ expansion came in.

From today, anyone who has a car that does not comply will have to pay a £12.50 daily fee in both outer and inner London.

Critics of the scheme say it penalises the poorest most - those who cannot afford to upgrade their cars and who need them to get to work and to see elderly relatives, for example.

Ms Kissi-Debrah, who has been working on the scheme with London mayor Sadiq Khan, admitted some people had "slipped through the net, for example care workers, evening shift".

She added: "Every new scheme that starts, there are always bumps in the road. We will have a conversation about that."

Read more:
Where expanded ULEZ will cover, why Sadiq Khan created it and why it is unpopular with some
Vehicle emission zones including ULEZ have made £418m across England since 2021
Protesters against ULEZ expansion stop London traffic

Asked what she would do if a future London mayor reversed the scheme, she told Sky News she would "have to go back to court".

She continued: "It is very clear that Sadiq Khan has a legal responsibility on air quality. He was advised by every single expert in Ella's inquest that this needs to go London-wide."

Ms Kissi-Debrah said she has support from England's chief medical officer, Professor Sir Chris Whitty.

"That's what his annual report was on last year - the impact of air pollution on the health of the nation," she said.

"And that's what drives me forwards still."