The European Court of Justice decided Bulgaria had breached EU limits for air pollutants.
The case was initiated by the European Commission. The EU court has found Bulgaria guilty for systematic and constant exceeding of EU norms on fine particulate matter on the territory of the whole country.
Small dust particles blamed for breathing problems and heart disease – recorded at several sites in Bulgaria from 2007 to 2014- showed severe breaches of EU clean air quality rules.
Environmental groups say the ruling will help in their efforts to force countries to take action over poor air quality, although there were no fines announced.
Apart from Bulgaria, the European Union executive has begun legal action against 15 countries over breaches of air quality standards, including particulate matter. The ECJ is currently reviewing a case against Poland.
The ruling, the first time the Court has endorsed EU air pollution rules, was seized upon by campaigners as a precedent for other cases.
Bulgaria will now have to work with the European Commission to improve the situation.
The Commission has estimated premature deaths attributable to air pollution in 2013 at 13,700 in Bulgaria and around 400,000 in the EU as a whole.
According to the World Health Organization, in 2012 air pollution caused 3 million premature death cases worldwide, including 479,000 in Europe and 8,634 in Bulgaria.