AMSTERDAM, Sept 16 (Reuters) - Amsterdam's Schiphol airport will reduce daily passenger numbers by 18% until at least Oct. 31 due to labour shortages, it said on Friday.

Schiphol, one of Europe's busiest airports, continues to grapple with long lines mainly due to a shortage of security staff, leading to some travellers missing their flights.

"This decision is bad news for passengers and for airlines," Hanne Buis, chief operating officer of the Royal Schiphol Group, said in a statement. "The reduction is necessary to guarantee the safety of our passengers and employees."

Airline KLM, which is part of Air France-KLM, said Schiphol's decision was "disappointing".

The airport said its decision would reduce average passenger numbers through the airport to 54,500 a day for the rest of September and to 57,000 in October.

On Thursday, the airport's chief executive, Dick Benschop, handed in his notice after measures he took to overcome the crisis proved insufficient.

Travel at Schiphol has been disrupted on and off since April. Hundreds of flights have been cancelled, and queues have become routine. (Reporting by Charlotte Van Campenhout and Bart Meijer; Editing by Andrew Cawthorne and Susan Fenton)