The judge said the walkouts by 190 staff who guide planes in and out of parking places, which have resulted in the cancellations of thousands of flights, were illegal and that the workers had a duty to keep industrial peace.

The ruling may now trigger compensation demands from German flagship airline Lufthansa (LHAG.DE), which has lost tens of millions of euros in revenue and seen bookings wane as a result of the strike.

Board member Stefan Lauer had said on Tuesday that if the strike were declared illegal, the airline would consider making a claim, as it has done in the case of a threatened strike by air traffic controllers last summer.

The GdF union, which said it would appeal the court ruling, is demanding higher pay and shorter working hours from airport operator Fraport (FRAG.DE) for the staff.

A second attempt at pay talks ended acrimoniously on Friday night, with Fraport making a lower offer than before the mediation process and the union saying it would no longer give 24 hours' notice of strikes.

Fraport was able to keep the effects of the strike under control by using former apron staff, thus ensuring around 80 percent of the airport's 1,300 daily flights could run.

Fraport and Lufthansa, which was the hardest hit by the strike at its home base, finally went to the courts after the union asked air traffic controllers to join the walkouts, which would have brought the hub to a standstill.

That move was blocked by the court in Frankfurt late on Tuesday night, but not before dozens of long-haul flights were cancelled or delayed.

In all, 235 flights were cancelled on Wednesday out of a total of 1,260, most of them owing to the airfield walkouts which were due to continue until 0400 GMT Thursday. Around 1,800 flights have been cancelled in total since the walkouts began.

Lufthansa said after the court ruling that flight operations could now start returning to normal, although it was too late to reinstate Wednesday's cancelled flights and there would likely be some delays and cancellations on Thursday as planes return to their scheduled positions.

Adding to passengers' woes, services union Verdi on Wednesday threatened strikes of ground staff at Berlin's airports Tegel and Schoenefeld in a wage dispute there and said it would give only about 15 minutes advance warning.

Frankfurt is Europe's third busiest in terms of passenger numbers after London Heathrow and Paris Charles de Gaulle.

(Reporting by Peter Maushagen and Victoria Bryan; additional reporting by Christian Ruettger; Editing by Jodie Ginsberg)

• A Permanent Economic Emergency