The lions were probably part of the king's private collection

It also shows the relationship between England's early monarchs and the "king of beasts" was not just a symbolic one.

The lions may have been among the first to turn up in Northern Europe since the big cats went extinct in the region at the end of the last Ice Age.

The menagerie was a popular tourist attraction, hosting exotic animals.

Lions were particularly prized as the living emblems of the royal arms of England, much like modern mascots Jeremy Ashbee, English Heritage

It was established by King John, who reigned in England from 1199-1216, and is known to have held lions and bears. It was finally closed in 1835, on the orders of the Duke of Wellington.

The Tower of London was once home to the Royal Menagerie

The remaining animals were moved to the Zoological Society's Gardens in Regent's Park, now known as London Zoo.

"The menagerie seems to have been a private collection for the king, a sign that he enjoyed good relations with foreign monarchs, who presented him with animals," said Jeremy Ashbee of English Heritage, curator of the Tower of London.

"Lions were particularly prized as the living emblems of the royal arms of England, much like modern mascots."

Lion tower

The big cats found at the Tower are thought to have been kept in the menagerie, but its actual location on the grounds is still unknown.

Edward I had a semi-circular structure - later known as the "lion tower" - built in the south-western corner of the Tower in 1276-7. By the 16th Century, historians know that this housed the Royal Menagerie.

"Finding two virtually complete big cat skulls dating back to the 13th and 15th Centuries from the moat adjacent to the Middle and Lion Towers suggests they were kept in this area," said project leader Dr Hannah O'Regan of Liverpool John Moores University.

Unearthed in the 1930s, the big cat skulls have been stored at the Natural History Museum for the last 70 years.

Unceremonial end

In addition to the lion skulls, the researchers also analysed a leopard skull and the skulls of 19 dogs.

The best preserved lion skull was radiocarbon dated to between AD 1280 and 1385, making it the earliest Medieval big cat known in Britain. The period when it lived covers the reigns of Edward I, II and III, when the lion tower was built.

The second lion skull was dated using the same method to AD 1420-1480. The leopard skull, which was badly damaged, dated to between 1440 and 1625, which covers the Plantagenet reign, the Tudors and Stuarts.

Despite their royal status, the cats were not treated with ceremony when they died, instead being dumped - unskinned - in the Tower's moat.