BERLIN, Germany — GdF, the German labor union organizing this week's strike at Frankfurt Airport, has called on air traffic controllers to join the walkout, a move which could disrupt hundreds more flights.
More from GlobalPost: Frankfurt Airport braces for new round of strikes
According to The Local, only 12 controllers would need to leave their posts for hundreds of flights in and out of Frankfurt to be grounded. "If all take part, we would think that actually nothing would fly," GdF union chairman Markus Siebers said.
Airport operator Fraport condemned the "massive disruption" a solidarity strike would cause. "This course of action is absolutely irresponsible," spokesman Mike Schweitzer told German TV, cited by the Associated Press.
Germany's air safety agency, DFS, has urged air traffic controllers not to join the industrial action, saying that to do so would be "irresponsible, inappropriate and disproportionate," the Wall Street Journal reported.
Both DFS and Fraport said they were examining whether legal action could be taken to prevent unions widening the strike. German air traffic controllers are employed by a state-owned group, so the companies could turn to the government in a bid to stop them participating.
GdF's latest walkout began on Sunday evening and will continue until Thursday morning. Some 190 ground staff are taking part, in a bid to increase pay and reduce working hours.
So far, Fraport has managed to keep 80 percent of flights running. The airport's website says today that operations are expected to run smoothly, though several domestic flights are listed as cancelled.
German airline Lufthansa said it would cancel around 65 European and domestic flights on Tuesday, but expected to operate all long-haul flights this week as scheduled.
Frankfurt Airport is Germany's main hub and Europe's third-busiest airport after London Heathrow and Paris Charles de Gaulle. The strike is estimated to cost Fraport around €1 million ($1.3 million) in lost revenue each day.