Came across this when I was googling--
The author Michael Thornton says that Barbara Cartland told him that her daughter Raine (the stepmother of Princess Diana) wasn't fathered by her husband. She said that was pretty impossible. The next candidate was her lover, the Duke of Portland, but he was believed sterile. So, "Cartland's third lover was not married and the most famous of the three. Handsome, debonair and bisexual, His Royal Highness Prince George, later the Duke of Kent, was the youngest surviving son of King George V and Queen Mary, the uncle of the present Queen, and the lover of Noel Coward. 'PG, as I called him, was 25 at that time and absolutely adorable,' said Cartland, 'as well as being the most amazing lover. In my heart, I have always believed that he was Raine's father. I was shattered when he, too, died in a plane crash, while on active service during the war.'"
Thornton revealed this in an interview in 2008 following a TV movie on Barbara Cartland and her first marriage. (She also had a rumored romance--which she shot down as untrue but which she seems to have admittedly enjoyed the rumor being tossed around for fun--with Earl Mountbatten.)