Officials at the Tower of London have uncovered a 900-year-old secret which means that the history of Britain's most famous tourist attraction will have to be rewritten.
It has been discovered that the White Tower, the imposing centrepiece of the Tower, was originally only two thirds the size it is now.
It emerges that the roof of the famous building is not the original built by William the Conqueror, as has always been assumed. Instead, in what might be the first example of a royal offspring trying to emulate and outdo his father, his son William Rufus put on a higher roof.
Guy Wilson, Master of the Armouries at the Tower of London, described the discovery as a "sensational" find. "It changes the history of the White Tower, one of the most famous buildings in Britain," he said.
The discovery was made by Geoffrey Parnell, Keeper of Tower History and author of the official history of the building. He was clearing parts of the White Tower to redisplay it after moving objects up to the new Royal Armouries museum in Leeds, when he found on a stone wall a visible scar of the original pitched roof.
A search through the Anglo- Saxon Chronicle showed there had been a hurricane in 1090, leaving the building "sore shaken by the wind".
The subsequent rebuilding of the Tower of London by William Rufus means that historians will now have to decide why he changed his father's plans, and what uses would have been made of the original, much smaller building. Mr Parnell said the significance for architectural historians was enormous.
He added: "There is a clear scar of a pitched roof which predates the flat one. It sets the cat among the pigeons in terms of the Tower's history. It now looks like the violent storm of 1090 was the sort of event which may have brought about a major remodelling of the building.
"Rufus was a builder monarch. He built the Great Hall at Westminster. He wasn't a man to do things by half. So he clearly decided to improve on his father's efforts. One has to accept that there's strong evidence of an earlier, lower roof, so we now have to find out how the earlier rooms worked. The two main chambers on the first floor now begin to look more like conventional Norman halls."
Mr Wilson added: "No one had thought before that the building was at the wrong height."
Mr Parnell's official book on the Tower says: "The White Tower is still one of the potent symbols of Norman authority. To the native Saxon population of London, unfamiliar with buildings of such scale and appearance, it must have provided a vivid reminder that a new order had been established."
Now, of course, the new discovery makes it clear that William the Conqueror's building was a third less imposing.
"It's an enigma," said Mr Parnell yesterday. "Why did Rufus do it? The original building would have looked different and worked differently. What went on inside it? We don't have the answers yet."
The re-examination of the White Tower, which attracted 2.3 million visitors last year, is leading to other discoveries. An exploration of disused chimney flues has found a bird's nest believed to be hundreds of years old.