"This strike is becoming increasingly out of proportion, which is why we have decided to apply for a temporary injunction against the solidarity strike," Fraport labour executive Herbert Mai said.

The union has asked 12 air traffic controllers to join the strike, now in its third week, from 0400 GMT to 1000 GMT on Wednesday, which would affect at least 300 flights, it said.

"The step is meant to show solidarity with the GdF members in the struggle against Fraport," the union said in a statement.

Walkouts by 190 employees, who guide planes to parking places and make up just 1 percent of Fraport's workforce, started on February 16 and have so far forced the cancellation of about 1,400 flights, with a further 200 expected on Tuesday.

The impact has been limited however, as Fraport (FRAG.DE) has been able to fill the gaps by reshuffling its workforce, meaning around 80 percent of daily scheduled flights have run. The airport averages 1,300 flights a day.

A strike by control tower staff would paralyse the airport as it would mean no flights could take off or land.

The DFS air traffic controllers' authority urged its Frankfurt staff not to join the strike, and said it, too, was considering legal measures.

"It cannot be allowed that the DFS, which is a state-owned company and holds a state-guaranteed monopoly, should be dragged into a private pay row by the GdF," Fraport said.

Along with the application for an injunction against the solidarity strike, Fraport and Lufthansa (LHAG.DE) have also applied for a temporary injunction against the GdF to halt the current and any further strikes by the apron workers.

The court will hear that case on Wednesday at 0800 GMT.

The latest walkouts began on Sunday at 2000 GMT and are due to run until 0400 GMT on Thursday.

OUT OF PROPORTION

Expanding strikes for solidarity reasons is allowed under German law as long as industrial action remains within reason.

"In our view, the expansion of the strike to air traffic controllers is not proportional as only a small group of Fraport's employees is involved in the tariff conflict," Silvia Quandt analyst Stefan Kick said.

The union wants higher pay and shorter hours for the staff who work on the airport apron. Fraport says the demands equate to pay hikes of 50-70 percent and are unreasonable.

The walkouts have already brought calls for changes to Germany's labour laws to prevent small groups of striking workers from exerting such pressure.

Fraport had so far refrained from legal measures, with Chief Executive Stefan Schulte saying last week it would only inflame the situation.

The airport operator on Tuesday also repeated its call for the German government to stop air traffic controllers -- who work for government-owned DFS -- from joining the strike.

"The threat to extend the strike would ... lead to economic damage that can under no circumstances be justified," Fraport said. Each day of strikes by the airfield staff costs Fraport around 1 million euros in lost revenue.

Strikes threatened last year by the GdF's tower workers across Germany would have disrupted around 3,000 flights a day but were averted by legal action and the intervention of the federal transport ministry.

Lufthansa, which accounts for around half of the airport's take-offs and landings and has been worst hit by the strike, said it was cancelling around 130 flights on Tuesday.

(Reporting by Victoria Bryan, Peter Dinkloh and Axel Hildebrand; Editing by Mark Potter, Greg Mahlich and Jane Merriman)

• A Permanent Economic Emergency