Dutch King Willem-Alexander has admitted to working as a co-pilot for over two decades. The King of the Netherlands regularly flew commercial planes, without his passengers' knowledge, he told Dutch newspaper De Telegraaf.

He said that he had been flying a Fokker 70 for Dutch airline KLM twice month for the past 21 years, often with Captain Maarten Putman.

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When he acceded to the throne in 2013 following his mother's abdication, Willem-Alexander carried on flying planes.

"I find flying simply fantastic," he told the newspaper, describing his monthly flights as a "hobby which requires complete concentration."

"You have a plane, passengers and crew and you are responsible for them," he said. You can't take your problems with you off the ground. You can completely switch off for a while and focus on something else."

He previously said that if he had not been born into the royal family, his dream would have been to fly large planes.

He always kept his identity a secret from passengers. "I warmly welcome passengers on behalf of the captain and crew," he said. "Then I don't have to give my name."

Since 9/11 the cockpit door is locked and the pilots have little contact with passengers, he added.

He said he was rarely identified in his pilot uniform, but admitted that some people had recognised his voice.

Willem-Alexander said he is now learning to fly Boeing 737s as KLM is phasing Fokker planes out. He is enthusiastic about his future as a co-pilot and plans to fly for many more years.

"It also seemed nice to fly to other destinations one day, with more passengers and bigger distances," he said. "That was the real motive for training on the 737."