The mayor of London, Sadiq Khan, has written to the attorney general asking him to back a new inquest into the death of a nine-year-old girl whose severe asthma attacks coincided with spikes in air pollution.

The mother of Ella Kissi-Debrah has fought a long campaign to highlight the role she believes illegal air pollution played in her daughter’s death in 2013.

In July, an expert in respiratory medicine backed her campaign, stating there was a “striking association” between Ella’s hospital admissions and the most severe episodes of air pollution around her home in south London.

Khan is calling on the attorney general, Geoffrey Cox, to authorise a new inquest “to ensure that the most ambitious measures are taken at every level of government so that – if air pollution was the cause – no other child ever again dies as a result of the air they breathe”.

The attorney general will only authorise a new inquest if he is satisfied there is “sufficient admissible evidence that there is a reasonable prospect of the high court being persuaded to order a new inquest”.

A spokeswoman for his office said Cox expressed his “sincere sympathies to Ella’s family”.

She added: “I can confirm that an application for approval to apply for a fresh inquest has been received by the attorney general’s office regarding Ella’s case and we will review the evidence.”

Ella’s mother, Rosamund Kissi-Debrah, said she was extremely grateful to the mayor of London, who is an asthma sufferer himself, for his support for the family’s calls for a fresh inquest.

“Since Ella’s death I have become so much more aware of the deadly threat posed by air pollution. A second inquest has the potential to help us better understand this threat and prevent other mums and dads having to go through the trauma of losing a child to this invisible killer,” she said.

The family’s lawyer, Jocelyn Cockburn of Hodge Jones & Allen, said it was an important moment in the family’s fight for justice.

“The attorney general will find it extremely persuasive that the mayor of London, who himself has air quality duties, has added his weight to the call for a new inquest into Ella’s death.”

She added: “As the mayor of London indicates, there is a real need to understand what role air pollution played in this child’s death – not least to learn lessons to ensure that other children do not suffer the same fate.”

New evidence has been submitted to the attorney general by Stephen Holgate, a professor of immunopharmacology at the University of Southampton. In it, he states there was a “real prospect that without illegal levels of air pollution Ella would not have died”.

Holgate mapped Ella’s admissions to hospital against spikes in air pollution levels around her home. The serious episode that culminated in her death on 15 February 2013 coincided with one of the worst air pollution surges in her local area.

He said nitrogen dioxide levels, primarily from diesel vehicles around Ella’s home near the notoriously busy south circular road were consistently above the legal limit of 40µg/m3. He gave his “firm view” that Ella’s death certificate should reflect air pollution as a causative factor.

“The dramatic worsening of her asthma in relation to air pollution episodes would go a long way to explain the timing of her exacerbations across her last four years,” Holgate said. “There is a real prospect that without unlawful levels of air pollution Ella would not have died.”

A cause of death linked to air pollution in Ella’s case would be a legal first. As concern over the threat from toxic air and the government’s failure to act grows, increasing numbers of people are considering legal action both in the workplace and against the state.

Air pollution, labelled a public health emergency by the World Health Organisation, leads to the premature deaths of at least 40,000 people a year in the UK – 9000 in London.

It is known to be a major risk factor for childhood asthma and other respiratory conditions and there is also a growing body of evidence linking toxic air to a range of other deadly conditions from heart disease to Alzheimer’s, Parkinson’s and dementia.