The Duchess of Cambridge, 36, is famous world-wide for her glossy hair, but underneath the well kept locks lies a large scar.
Kate often wears her hair up in a way which hides the scar, and it had successfully remained unnoticed for a long time after her marriage to Prince William.
It was only a few years ago that the alarm was raised by concerned fans who had spotted the white gash hidden beneath her hairline.
Until then the mark been mistaken for hair extensions.
Worried commentators speculated that Kate might have undergone emergency surgery to have a tumour removed.
To put the fears of royal watchers to bed, Buckingham Palace released a statement where they said: “The scar is related to a childhood operation.”
Senior royal sources claimed the operation had been very serious.
Now an expert surgeon has suggested the scar is likely to be left over from a birthmark.
John Scurr, a consultant surgeon at the Lister Hospital in London told the Daily Mail: “I really doubt it was any serious medical condition.
“I would say it is as a result of an arteriovenous malformation – a birthmark – being removed.”
Dr Scurr expressed his bemusement that people had failed to spot such a scar for so many years.
He said: “It is remarkable, given the measurement, that no-one has noticed before.”
Kate may have been concealing her scar from the public, but she would have no need to hide the mark while at home with the Duke of Cambridge, who sports his own notable scar.
Prince William, 36, has a mark on his head which he jokingly calls his “Harry Potter” scar.
It is said that he acquired the scar at the tender age of 13 after being hit with a golf club.