Salisbury Cathedral is home to one of four surviving copies of the historic document, the Magna Carta. Issued by King John in 1215 as a solution to a political crisis he was facing.
It established for the first time the monarch was subject to the law of the land, rather than above it.
Although nearly a third of the text was dropped or substantially rewritten within ten years and almost all the clauses have been repealed in modern times, Magna Carta remains a cornerstone of the British Constitution and its principles are echoed in the US constitution and others around the world.
As well as providing a once-in-a-lifetime opportunity for researchers to study the documents together, the unification will be a chance for 1,215 members of the public to see the manuscripts side-by-side after winning free tickets in an public ballot.
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Mostly dry and bright this afternoon with cloud building and rain arriving from the west later.
A risk of heavy downpours in parts of Kent this afternoon. Elsewhere, mostly dry with sunny spells.
Thunderstorms and heavy downpours caused flooding across the Meridian region yesterday. Some residents have been evacuated from their homes.