The EU's top court on Thursday found Poland guilty of violating air quality laws, in the latest clash between the bloc's authorities and the rightwing government in Warsaw.
The European Court of Justice urged Poland to comply "without delay" or face fines after finding it had exceeded limits for toxic fine particles for years.
Brussels has repeatedly taken Poland's Law and Justice (PiS) government to task in recent years over other environmental issues as well as migration and the rule of law.
"The Court of Justice finds that Poland has infringed EU law on ambient air quality," the Luxembourg-based court said in a statement.
It said that between 2007 and 2015, Poland regularly exceeded the daily limits for fine particles in the air in 35 zones and the annual limit in nine zones.
The tiny particles may contain hydrocarbons, heavy metals, and dioxins that can enter the respiratory tract and the lungs.
The European Commission, the executive of the 28-nation EU, took Poland to the top court for failing to comply with an EU directive that took effect in June 2008.
The court said Poland could not use its "socio-economic and financial" arguments to justify setting late deadlines of between 2020 to 2024 to comply with the laws.
A 2016 World Health Organization report revealed that 33 of Europe's 50 most polluted cities were in Poland, a major coal-burning nation.
50,000 premature deaths
The European Environmental Agency blames air pollution for an estimated 50,000 premature deaths per year in the country of 38 million.
The commission has already taken Bulgaria to the top court which found it in breach of air quality standards.
And it has warned it could also pursue legal action against France, Germany, Britain, Spain, Hungary, Italy, the Czech Republic, Slovakia and Romania for violating air quality standards.
Air pollution is blamed for around 400,000 premature deaths in the bloc every year.
Thursday's ruling comes two days after the top advisor to the ECJ said Poland broke the law by logging in one of Europe's last primeval forests.
The European Commission in December launched unprecedented disciplinary proceedings against Poland over judicial reforms which Brussels says threaten the rule of law.
It has also taken Poland, along with Hungary and the Czech Republic, to the top court for failing to compy with a Brussels decision to relocate asylum seekers from Italy and Greece.
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