LONDON — The Tower of London's legendary ravens have been moved indoors to protect them from the threat of bird flu, the man in charge of the royal birds said Monday.

"Although we don't like having to bring the Tower ravens inside, we believe it is the safest thing to do for their own protection, given the speed that the virus is moving across Europe,'' said Derrick Coyle, the Tower's raven master.

According to legend, if the ravens leave the 11th century fortress on the River Thames, its White Tower will crumble and the Kingdom of England will fall. King Charles II decreed in the 17th century that there must always be six ravens at the Tower.

Tower officials said the six birds were taken inside as a contingency measure and that they'll be living in custom-built aviaries. The ravens are named Branwen, Hugine, Munin, Gwyllum, Thor and Baldrick.

The officials said the Tower had been monitoring the virus since it first appeared in Asia and had been planning to take the ravens inside as soon as it appeared in western Europe.