The UK government is being sued for a third time over the widespread illegal levels of air pollution, which cause 40,000 early deaths every year.

Environmental lawyers ClientEarth have already defeated ministers twice in court, forcing a new pollution plan to be drawn up in July. But ClientEarth believes even the latest strategy does not meet the legal requirement of banishing toxic air in the “shortest possible time”, as EU law requires.

“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,” said James Thornton, ClientEarth’s CEO. “We need clarity from the government and for that we’ve been forced to go back to court.”

ClientEarth sent ministers a pre-action legal letter setting out their concerns in October but deemed the government’s response inadequate. They have now applied to the courts for a legal hearing. The government has already spent £370,000 of taxpayers’ money in failed attempts to fight air pollution court action.

Nitrogen dioxide pollution, mostly produced by diesel vehicles, has been illegally high in most urban parts of Britain since 2010. The government’s latest plan was condemned as “woefully inadequate” by city leaders and “inexcusable” by doctors.

Air pollution causes an estimated 23,500 early deaths every year from NO2, rising to 40,000 when other pollutants are considered. In September, the UN’s special rapporteur on pollution said the government was “flouting” its duty to protect the lives and health of its citizens and in October a major pollution report estimated the number of premature deaths in the UK at 50,000 per year.

Oliver Hayes, at Friends of the Earth, said: “It’s shameful that the government has to keep being taken to court to try to force it to protect the health of its citizens.”

ClientEarth believes there are several grounds for judicial review, including backtracking in the latest plan on “clear air zones” in Birmingham, Derby, Leeds, Nottingham and Southampton. These zones, which would use charges to deter polluting vehicles from city centres, were mandatory in previous plans but are now only “expected” to be implemented.

ClientEarth also say it is unacceptable that the plan requires no action in 45 local authorities with illegal levels of air pollution. These include Leicester, Oxford, Liverpool, Cheltenham and Sunderland, with the government arguing toxic air will fall to legal levels without enforced action.

However, Leicester and Oxford city councils have written to ministers saying that the government has seriously underestimated pollution levels and that by requiring no action, they are stopping the councils getting access to funding to cut pollution. Oxford is planning to ban all petrol and diesel vehicles in the future.

Some motoring groups are campaigning against a crackdown on diesel cars, but the latest data shows car buyers are abandoning the technology, with sales down by 30% in October year-on-year. The government’s own analysis shows charging zones to deter dirty cars are by far the most effective policy but ministers have told councils they should only be the option of last resort.

“It’s time ministers came clean about the size of the problem and the difficult decisions needed to solve it,” said Thornton. He said the forthcoming budget should use tax changes to make diesel cars less attractive and that the motor industry should be made to contribute funding.

“The car industry helped get us into this mess so they should be helping get us out of it by contributing to a clean air fund, as they have done in Germany,” Thornton said. The German government recently secured €250m (£220m) from the car industry to help cities reduce pollution.

A spokeswoman for the Department for Environment, Food and Rural Affairs said: “We have put in place a £3bn plan to improve air quality and reduce harmful emissions. We will also end the sale of new diesel and petrol cars by 2040, and next year we will publish a comprehensive Clean Air Strategy which will set out further steps to tackle air pollution.”

The decision to take the government back to court came as four air pollution protesters were jailed for staging a series of direct action demonstrations against toxic air in the capital.

The fouractivists are from the environmental group Stop Killing Londoners, which has been stepping up its campaign calling on the government and the mayor of London to do more to tackle air pollution.

Camberwell magistrates court heard on Tuesday that the group had spray-painted “Cut Air Pollution” on City Hall on Monday night before sitting down and waiting to be arrested for criminal damage.

Roger Hallam, 51, Stuart Basden, 34, Ian Bray, 50, and Genny Scherer, 71, were under strict bail conditions not to go within 50m of the building after they graffitied it earlier this week. The four all deny criminal damage.

In a statement released after the hearing, the group said at least two of those imprisoned would go on hunger strike while being held on remand.

It said Monday’s arrests were the fifth action in the past week by the campaign amid growing anger at the air pollution crisis. They have demanded a meeting with environmental secretary Michael Gove and London mayor Sadiq Khan.

In a statement they added: “Today, the four of us are being sent to prison because we apparently care too much. We care about the 25 people who are killed every day in London by the illegal levels of air pollution. We care about the children who will endure a lifetime of suffering due to shrunken lungs. We care, because each breath we take is harming us, and puts even more strain on the NHS as it struggles to cope with unnecessary cuts.”

The chairwoman of the bench, Finola Gowers, remanded the protesters in custody for seven days and said they faced a possible custodial sentence if convicted.