The European Parliament was overruled by the highest EU court today over its attempt to reduce the number of sessions it holds in the French city of Strasbourg.
The European Court of Justice (ECJ) said the proposed change went against EU treaties and it was up to national governments, not the parliament, to make such decisions.
EU treaties oblige the parliament to hold 12 plenary sessions a year in Strasbourg, 430 kilometres (270 miles) from Brussels, which is home to the European Commission, the EU executive.
The Strasbourg seat costs nearly 200 million euros ($260 million) a year in travel and running costs, and the parliament decided to squeeze two Strasbourg sessions together in 2012 and 2013, effectively reducing the annual totals to 11 sessions.
France challenged this change because it regarded it as a threat to the status of Strasbourg, where France wants to keep a major EU institution and the business that goes with it.
The court's ruling on the proposed change is a blow to the parliament's authority because it stresses the primacy of national governments in such matters.
"It is not for the parliament or the court to remedy that situation," the ECJ said in a statement. "Rather, it is for the member states to do so, if appropriate, in the exercise of their competence to determine the seats of the institutions."
Some campaigners - mostly Dutch and British parliamentarians - emphasise the cost and inconvenience of doing the same job while switching between two locations. The Strasbourg building stands almost vacant for more than 300 days a year and accrues 12 million euros in maintenance costs.
British conservative member Ashley Fox, who proposed the change, said the ECJ ruling was undemocratic and he would now press EU member states to abolish the current arrangement.
"This case is no more than a skirmish in our longer-term war to stop the ridiculous two-seat travelling circus altogether," he said in a statement. "Every ordinary member of the public I meet wonders how this ridiculous situation just can't stop."
Beyond the planned reductions in Strasbourg sessions this year and next, parliamentarians also voted this year for a single seat on a permanent basis.
The Greens/EFA (European Free Alliance) parliament group stress the environmental damage from the monthly travel by the parliament's 754 members and more than 3,000 staff. This leads to an excess 20,000 tonnes of CO2 being emitted every year, according to a study commissioned by the group.
"The European Parliament operating arrangement has sadly become a symbol of waste in the EU," the Greens/EFA said in a statement on Thursday.
The parliament is now expected to draw up a new timetable for 2013, including 12 full sessions in Strasbourg, party officials said.