Low-cost airline Ryanair on Thursday lost an EU court battle for the right to impose Irish labour laws on its cabin crew working elsewhere in Europe.

The European Court of Justice, the bloc's top court, ruled in favour of six Ryanair workers of different nationalities based at Charleroi airport in Belgium, who argued that they should be subject to Belgian employment law.

The no-frills carrier considered their work to be done in Ireland because the planes they worked on were registered there, though the contracts stated their "home base" was Charleroi, where they started and ended their working days.

In its judgment, the court said European rules on legal jurisdiction were aimed at protecting the weaker party and gave the employee the right to sue their employer in the courts "closest to his interests" -- either the country where the employer is based or where the employee actually works.

A clause in the crew's contracts stating stating that Irish courts had jurisdiction and seeking to prevent employees bringing proceedings elsewhere was not enforceable, the court ruled.

Ryanair announced a year-on-year boom of 55 percent in quarterly profits in July, earning 397 million euros ($463 million) before tax, though pugnacious Chief Executive Michael O'Leary has warned of the risks posed by Britain's impending departure from the EU.