Wilhelm-Karl von Preussen often and fondly talked of his last visit to his grandfather, the ex-Kaiser Wilhelm II, in Doorn in the Netherlands where he lived out his years in exile.

Von Preussen had gone to see him in the summer of 1940 as a cadet on leave from Hitler’s army. Hermann Goering, commander of the Luft-waffe, had just launched the cross-Channel air assault that became the Battle of Britain. “He has no hope of winning that one,” the 81-year-old ex-Kaiser confided to his 18-year-old grandson.

From the age of 5, von Preussen, a great-great-grandson of Queen Victoria, had spent ten days every summer at his grandfather’s home. “He had the great gift of making every one of his grandchildren feel special,” von Preussen recalled.

The ex-Kaiser died a year later, and von Preussen’s privileged position as a royal was ended by Hitler during the war. He had enlisted in the German Army at l7, as war was breaking out. He was commissioned as a lieutenant and served in Russia. But his military career was brought to an abrupt end in l943 when he was thrown out of the army as a “royal prince” – a decree by Hitler stripped all former members of the Hohenzollern and Habsburg dynasties of their commissions.

Von Preussen went back to one of the family estates in East Prussia to learn estate management. But as the Soviet armies overran East Prussia in l944, he fled to the West. He ended up in Holzminden, a small town near Hanover, and there spent the rest of his life.

To earn a living, von Preussen went to work for a local chemical company that produced ingredients for perfumes. Eventually he became its managing director.

Alongside his new professional, middle-class life, Wilhelm-Karl continued to feel a duty to his royal past. He sought to discharge it by service to Die Johanniter, the Knights of St John, a significant charitable institution in Germany.

He had joined the Knights in 1944, at the age of 22, and at the age of 36 became the order’s effective executive director, an honorary and part-time post, serving until he was 77. He then retired to become its “Protector”.

Under his leadership, the Knights of St John increased to 50,000 workers, mostly volunteers, but with a hard core of professionals among its ranks.

By the time von Preussen gave up his active management of the Knights, they were running 80 clinics, hospitals and homes for the elderly and 200 kindergartens, and they maintained a fast and highly trained ambulance service.

In a tribute to von Preussen the order declared: “The Prince led us with a firm hand and sure touch through an age of social and political upheaval.”

He is survived by his wife, Armgard, and by their two sons and a daughter.

Prince Wilhelm-Karl von Preussen, leader of the Knights of St John, was born on January 30, 1922. He died on April 9, 2007, aged 85