Queen Maria of Roumania's younger son (Pr.Mircea) and possibly her youngest daughter Ileana, could have been fathered by her lover Count Stirbey.
King Olav of Norway could not have been fathered by King Haakon, but by Q.Maud's gynecologist, after 7 years of a childless marriage and the absolute need of an heir.
Olav was the only son of the "couple".
Another royal cuckoo could be Isabel Sartorius' daughter, generally accepted she was fathered by the then Pr.of Asturias and not her hastly found husband, a cousin of the Albas through Cayetana's husband.
I know there are plenty of such examples in European history, but I was just wondering about the most likely and notable cases. Aside from the children of Queen Isabel II of Spain, I'm led to recall the youngest children of Grand Duke Ludwig IX of Hesse and by Rhine, who married his first cousin, Princess Wilhelmine of Baden. It is widely believed that Prince Alexander (founder of the morganatic branch known as the Battenbergs and Princess Marie (who became the empress consort of Russia, the dynastic first wife of Czar Alexander II) were not his biological children.
The second set of twins born to the late Prince Alexander Karageorgevich of Serbia (born in 1924) and his first wife, Princess Maria Pia of Savoy, are believed to have been biologically fathered by Prince Michél of Bourbon-Parma. And the youngest children born to Grand Duchess Maria Kirillovna of Russia (1905-1951) are not believed to have been biologically fathered by her husband, Karl, the 6th Prince of Leiningen.
And Empress Eugénie of the French (born to nobility, a member of royalty by marriage) is believed to have been biologically fathered by a man other than Cipriano de Palafox y Portocarrero, 8th Count of Montijo.
It is almost near certain that King Friedrich Augusts III of Saxony was not the biological father of Princess Anna Monika, the youngest child born to his wife, Archduchess Luise Austria, Princess of Tuscany.
As for King Edward IV of England ... that was so long ago, birth records are so murky, and I don't know what to make of the claim that Richard, the 3rd Duke of York, was not his biological father ... One is only too familiar with how stories originate and spread over the years, much of it for reasons of politics and propaganda ...
What are some other notable examples of royal cuckoos? Have there ever been questions raised over the paternity of James Stuart, who succeeded to the Scottish throne as King James VI and the English as James I? That perhaps Henry Stuart, Lord Darnley, was not his biological father?