The 300-year-old Treaty of Utrecht, which is currently in the British Library, was signed to end the war of Spanish succession and gives Britain the right to Gibraltar.
The treaty, which was seen today by ITV News Europe Editor James Mates, says the territory is Britain's "to hold forever, without any impediment or exception whatsoever".
Despite being signed on April 11, 1713, the document remains legal.
More top news
Fainting dogs, dancing grooms and an attacker who picked on the wrong girl all featured highly in our most-read stories of the year.
Some dogs have been at the Battersea Dogs and Cats Home for more than a year. Can you make their Christmas and give them a new home?
Selfless Donna Wood from Staffordshire has left her husband and children at home so she can help save lives in Sierra Leone.