"We will discuss this evening whether other union members, such as the tower controllers, will join the strike in a show of solidarity," a union spokesman told Reuters on Monday.

Walkouts by the employees who guide planes to parking places are now in their third week and have so far had a limited impact at Frankfurt, Europe's third-busiest airport in terms of passenger numbers.

Fraport (FRAG.DE) has been able to mobilise former airfield workers to fill in the gaps, meaning around 80 percent of the daily scheduled flights have run.

Should tower staff join in, it could mean chaos for the airport, which has an average of 1,300 flights taking off and landing each day.

A strike threatened last year by air traffic controllers across Germany would have disrupted around 3,000 flights a day, not only those taking off and landing in Germany, but also those flying over the country.

Airport operator Fraport expects around 200 flight cancellations on Monday at Frankfurt. Lufthansa (LHAG.DE), which accounts for around half of the airport's take-off and landings, said it was cancelling around 140 flights on Monday and a similar number on Tuesday.

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

Each day of strikes is costing Fraport around 1 million euros in lost revenue. Lufthansa said last week the first four days of strikes had cost it a high double-digit million euros amount, but has declined to provide a new estimate since.

The GdF union is calling for higher pay and shorter hours for around 190 staff who work on the airport apron. Fraport says the demands, which would equate to pay increases of between 50 and 70 percent, are unreasonable.

The walkouts by staff representing just 1 percent of Fraport's workforce have also resulted in calls for changes to Germany's labour laws to prevent such small groups of striking workers from exercising such pressure.

"A mini-union cannot be allowed to blackmail a company in this fashion," CDU/CSU politician Michael Fuchs said last week.

Fraport's own staff council has distanced itself from the striking workers, saying they were not acting in the best interests of the workforce as a whole.

The number of flights cancelled since the walkouts began on February 16 is around 1,400, including Monday's expected cancellations.

Shares in Fraport and Lufthansa fell on Monday, with Fraport down 1.2 percent and Lufthansa, also burdened by high oil prices, down 3.6 percent.

(Reporting by Victoria Bryan and Frank Siebelt; Editing by David Holmes and Erica Billingham)

• A Permanent Economic Emergency