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
Big Ben will fall silent on Monday for renovations following a backlash from MPs over the proposed four-year timetable for the work.
Rain will push into the northwest, turning heavy and persistent in places
Thousands of dangerous appliances are still in use across the UK despite years of warnings about the dangers.