A distressed Prince William phoned Princess Diana "upset" over cruel jibes from his Eton classmates following the scandalous publication of topless photos in a UK tabloid.
The 14-year-old William said other boys were teasing him over the size of his mother's breasts.
The revelation, from Conde Nast chairman Nicholas Coleridge, comes ahead of the publication of his memoirs, The Glossy Years.
Writing in the Daily Mail, Coleridge recalled Diana visiting him after the photos were published in the Daily Mirror tabloid.
Coleridge remembers Diana saying: "Nicholas, can I ask you something? Please be truthful. Did you see the photograph of me in the Daily Mirror? The topless one?".
According to Coleridge, she went on to describe the effect of the scandal on her oldest son, then a pupil at the prestigious school.
"He was so upset. He said some of the boys were teasing him saying my t**s are too small. Are my breasts too small, do you think?".
Coleridge reveals he went "breathless and as red as a guardsman's tunic" before reassuring Diana that her breasts were "perfect" and telling her not to worry.
William was to face his own paparazzi nightmare when a French magazine published topless photos of his wife.
French scandal sheet Closer sparked controversy after it published paparazzi snaps of the Duchess of Cambridge while she privately sunbathed with her husband in Provence at a chateau owned by the Queen's nephew, Lord Linley.
The blurry photos show the duchess wearing a skimpy bikini bottom and sunglasses as she relaxed on a terrace with William.
A royal spokesman said at the time that the photographs were a "grotesque invasion of privacy".
"Their Royal Highnesses had every expectation of privacy in the remote house. It is unthinkable that anyone should take such photographs, let alone publish them," said the St James' Palace official.
"The incident is reminiscent of the worst excesses of the press and paparazzi during the life of Diana, Princess of Wales, and all the more upsetting to the Duke and Duchess for being so."