The resurfaced recollection comes as Morgan is locked in an increasingly bitter dispute with Diana’s brother Charles Spencer, 62, over his new memoir, Swan Song: Diana, My Sister.
Spencer incorrectly claimed Morgan was partly responsible for the publication of intrusive photographs showing Diana exercising in a leotard at a gym.
The pictures were actually published by the Sunday Mirror in November 1993, when Morgan was working for rival newspaper The Sun.
He did not become Daily Mirror editor until October 1995.
Spencer has since apologized for the mistake, saying accuracy was important to him and acknowledging his assumption was incorrect.
Morgan, however, said the apology would not stop him pursuing legal action against Spencer and publisher Penguin.
He said: “Earl Spencer has apologised to me for his defamatory lies published in his new book, Swan Song, published today.
“But I am pursuing legal action against him and his publisher Penguin to have the lies removed, and damages paid to the King’s Trust.”