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
Outbreaks of rain will continue to spread northeastwards, turning most persistent and heavy in the southwest
Former shadow foreign secretary Hilary Benn led calls by a series of senior Labour MPs urging colleagues to 'win' back the party.
A team of detectives and specialists is set to begin a painstaking excavation of the area where Ben is thought to have gone missing in 1991.