Despite talking over the intercom to passengers for more than two decades, no one ever realised that the Dutch king was at the controls.

King Willem-Alexander said in an interview with De Telegraaf newspaper Wednesday that he had taken to the skies twice a month on short haul services.

The King of the Netherlands has revealed that for the past 21 years he has been operating as a pilot for the national carrier, KLM .

On staying undercover the monarch said he was never under obligation to say his name.

"The advantage is that I can always say I am speaking on behalf of the captain and crew to welcome them on board, so I don't have to say my name.

"But then, most people don't listen anyway," he said.

The 50-year-old royal said being able to leave his royal duties behind to concentrate on flying was "relaxing".

The King will take a break from his pilot duties in order to learn to fly Boeing 737s, KLM's upgrade of choice for its short haul fleet.