ClientEarth supporters and the Green Party's Caroline Russell outside court last year. Photograph: ClientEarth

ClientEarth supporters with London Assembly member Caroline Russell outside court last year. Photograph: ClientEarth

Hackney environmental law firm ClientEarth is taking legal action against the government for a third time over its “stubborn failure” to combat illegal air pollution, as it continues its fight to rid the UK of “poisoned playgrounds”.

The move comes just a year after the firm won a High Court battle that forced ministers to develop plans to tackle illegal levels of nitrogen dioxide across the country.

ClientEarth came out on top in another court scrap in April when a judge rejected ministers’ attempts to delay publication of the plans.

The firm’s CEO James Thornton said: “The UK Government’s stubborn failure to tackle illegal and harmful levels of pollution in this country means that we have no choice but to take legal action.

“We need clarity from the government and for that we’ve been forced to go back to court.”

ClientEarth is also campaigning to tackle toxic air around schools.

It launched a ‘Poisoned Playgrounds’ petition in September which calls for a “comprehensive network of Clean Air Zones” across the UK, as well as new legislation to protect people from pollution.

The campaign says studies have shown that the health of children living within 150 metres of illegally polluted roads can be affected by traffic-related air pollution, and asks: “Is your child playing in a poisoned playground?”

So far, more than 1,000 people have signed up to support the drive, which has a map tool on its website for people to check how close their local schools are to polluted roads.

A striking video accompanying the petition shows children running around a playground wearing heavy-duty gas masks.

ClientEarth revealed that at least 950 schools across the country are either on or near roads with illegal levels of dirty air.

Two primary schools in Hackney are soon to take part in London Mayor Sadiq Khan’s first “air quality audits”

William Patten and De Beauvoir are among 50 schools involved in the pilot, which aims to identify ways of protecting children from toxic fumes in the capital’s most polluted areas.

report last year revealed William Patten was one of 27 primary schools in Hackney where levels of toxic air breached EU limits.

You can find ClientEarth’s petition here.