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.
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Leicester City players have been celebrating their Premier League title after Spurs drew with Chelsea on Monday night.
Tonight western and northern areas will remain a little breezy with a few showers at times.
Leicester are Premier League champions after Tottenham conceded a two-goal lead in a bad-tempered 2-2 draw at Chelsea.