A fresh inquest will be held into the death of nine-year-old Ella Kissi-Debrah who suffered a fatal asthma attack believed to have been linked to air pollution near her home in south London.
Ella lived near one of the capital's busiest roads and died in February 2013 after three years of seizures and 27 visits to hospital for asthma attacks.
Lawyers previously said her original 2014 inquest did not investigate the potential impact of air pollution.
It concluded that Ella's cause of death was acute respiratory failure caused by a severe asthma attack.
But an expert report by Professor Stephen Holgate, previously quoted in a submission to the Attorney General, said that it was likely that unlawful levels of air pollution contributed to Ella's fatal asthma attack.
Her mother Rosamund applied to the High Court for a fresh inquest following new evidence which persuaded the Attorney General the original verdict of the coroner could be overturned.
A judge has now said "the discovery of new evidence makes it necessary on the interests of justice that a fresh inquest be held."
Ella may become the first person in the UK for whom air pollution is listed as the cause of death.
Family of girl, 9, who died due to 'illegal' air pollution near home apply for new inquest
In a statement after the ruling, her mother Rosamund said: "I am absolutely delighted by today's ruling and look forward to finally getting the truth about Ella's death.
"The past six years of not knowing why my beautiful, bright and bubbly daughter died has been difficult for me and my family, but I hope the new inquest will answer whether air pollution took her away from us.
"If it is proved that pollution killed Ella then the Government will be forced to sit up and take notice that this hidden but deadly killer is cutting short our children's lives."
Speaking outside the Royal Courts of Justice, Rosamund said she was "overwhelmed" by the "amazing victory".
She said: "It's a fantastic day for my family and my surviving children."
The mum added that she had "absolutely no doubt whatsoever" that Ella's death was related to air pollution.
She said she wanted "to show to the British public what air pollution can actually do - they need to start getting angry about this".
'Illegal levels of pollution killed my daughter - it's a public health emergency'
The report, obtained in April 2018, said air pollution levels at the Catford monitoring station one mile from Ella's home "consistently" exceeded lawful EU limits over the three years prior to her death.
Professor Holgate - a leading expert in asthma and air pollution - found a "striking association" between Ella's hospital admissions and air pollution episodes.
He concluded there was "a real prospect that without unlawful levels of air pollution Ella would not have died".
Mr Justice Jeremy Baker, QC, said: "Reliance is placed on an expert report by Professor Holgate, a professor in immunopharmacology and consultant in respiratory physician, which concludes that it is likely that unlawful levels of air pollution contributed to the cause and severity of Ella's fatal asthma attack, and that there is real prospect that without unlawful levels of air pollution, Ella would not have died."
"This report has led to further evidence being obtained which, it is summited, indicates that there was an arguable failure by the state to reduce the air pollution which was connected to Ella's death.
"In our judgment, the discovery of new evidence makes it necessary on the interests of justice that a fresh inquest be held."
A new date for the inquest is yet to be set.
Ella lived 25 metres from the South Circular Road in Lewisham - one of the capital's busiest roads.
The inquest at Inner South London Coroners' Court in 2014, focused on Ella's medical care and concluded the cause of her death was acute respiratory failure as a result of a severe asthma attack.
But High Court judges, in agreement with the Attorney General, have concluded the report of Professor Holgate linking air pollution to asthma attacks, impairs the original finding.
Attorney General Geoffrey Cox granted the family permission to apply for a fresh inquest in January of this year, after deciding there was 'new evidence which may alter the substantial truth of Ella's death'.
Ms Kissi-Debrah wept in court as she heard the decision.
Jocelyn Cockburn, partner at Hodge Jones & Allen, which represents Ms Kissi-Debrah, previously said 'We hope that the High Court will look favourably on this application and allow a further inquest into Ella's death.
"Only then will the family get the answers it needs to help them understand what led to Ella's death six years ago and whether it was avoidable.
"Furthermore, this case has wider implications - as in large areas of our country, children continue to breathe in unlawful levels of air pollution.
"This investigation will shine a light on what the Government is doing, or not doing, to clean up the air in the soonest time possible as is required by law and identify steps that must be taken to avoid future deaths."
The appeal before Lord Justice Irwin and Mr Justice Jeremy Baker and His Honour Judge Lucraft, QC, was granted.
