V. Froude, Elizabeth, vol. I, ch. iv,
letter of De Quadra to Philip of Spain:
In the afternoon we were in a barge, watching the games on the river. (The queen) was alone with Lord Robert and myself on the poop, when they began to talk nonsense, and went so far that Lord Robert at last said, as I was on the spot there was no reason why they should not be married if the queen pleased.
In full - James Anthony Froude, History of England from the Fall of Wolsey to the Defeat of the Spanish Armada (1856-1870). A work of twelve volumes where six pertain to the reign of Elizabeth.
James Anthony Froude (1818-1894) was an English historian. Alvarez de Quadra was the Spanish Bishop of Aquila and Ambassador to England. De Quadra's letters are archived with the Spanish State Papers. Philip II (1527-1598) was King of Spain (1556). Lord Robert was Robert Dudley (1532?-1588), later Earl of Leicester (1564). The Queen, of course, was Elizabeth I of England (1533-1603; crowned 1558). De Quadra's letter was dated June 30, 1561, making the Queen 27 years old at the time and Dudley probably 28.
This site has a copy of de Quadra's letter.