Romania could be fined for not transposing EU rules on sulphur emissions.
The EU Commission has asked the Court of Justice to impose a daily penalty worth more than €38,000 (£26,220).
The sulphur legislation aims to reduce the effects of the pollutant which is emitted by ships and harms human health, the environment and contributes to acid rain, it added.
Without the law, emissions from shipping would surpass combined emissions from all land-based sources, hindering many cities with ports in meeting the EU’s air quality legislation.
The proposed penalty takes into account the seriousness of the infringement and the duration of 17 months.
The Court will decide whether to fine Romania or not, however the penalty cannot exceed the amount specified by the Commission.
The European Commission is also taking Greece to the Court of Justice over poor wastewater treatment again.