The European Union's top court ruled on Thursday that France had failed to adequately prevent water pollution by nitrates and ordered the government to implement regulation or face penalties.

The European Commission took the French government to court for failing to prevent water pollution in vulnerable zones, including some of France's iconic shoreline.

Synthetic nitrate fertilisers are used in large-scale farming and have been linked to contaminated drinking water.

Nitrates can also cause rapid growth of algae that damages coastal marine life, a phenomenon that grabbed headlines in France when popular beaches in Brittany were infested by green slime at peak holiday time.

EU regulation requires that designated areas, including farm land near key waterways, should have closed periods when manure and chemical fertilisers cannot be used.

In the decision, the court ruled that France failed to implement these limitations, with moratoriums that were too short and by allowing storage of fertilisers in vulnerable areas that also broke EU rules.

After the EU issued its directive on the use of nitrates, "the French Republic failed to fulfil its obligations" on several fronts, the court said.

In addition, the court said that French regulation was too vague for farmers and local authorities to accurately measure the appropriate use of nitrates in controlled zones.

This is the second decision against France over nitrates, with a ruling last year finding authorities had only carried out an incomplete census to designate vulnerable zones.

The French government faces fierce resistance to limiting nitrate use from France's powerful agricultural lobby, which argues that EU nitrate regulation, drawn up in 1991, fails to account for new technology and methodology.