Prince Frederick of Prussia (German: Prinz Friedrich Georg Wilhelm Christoph von Preußen; 19 December 1911 – 20 April 1966), also known as "Mr. Friedrich von Preussen" in England, was the fourth son of Crown Prince Wilhelm of Germany and Duchess Cecilie of Mecklenburg-Schwerin.

On 30 July 1945, he married Lady Brigid Guinness, daughter of Rupert Guinness, 2nd Earl of Iveagh, at Little Hadham, Hertfordshire. They had issue:

He was studying at Cambridge, living incognito under the name of Count von Lingen, when war broke out in September 1939. He was arrested and interned in May 1940. He was held in England for several months, then sent to internment camps near Quebec City and, soon afterwards, Farnham, Quebec. In both camps, he was elected camp leader by fellow inmates.

He renounced his German citizenship in 1947. Being a descendant of Sophia of Hanover, and having rights under the Act of Settlement 1701, as amended by the Sophia Naturalisation Act 1705, he was naturalised as a British citizen in October 1947 under the name Mr. Friedrich von Preussen (having also been known during residence in the UK as "George Mansfield"). This naturalization was controversial to some, and his status and a subsequent claim for compensation was debated in Parliament and the law courts until 1961. In the period 1917-32, it was settled that a person who had a foreign title would normally undertake to relinquish it before he/she could receive a certificate of British naturalization, and no exception was made in the case of Mr. Friedrich von Preussen.

...
Wikipedia