A retired insurance broker has uncovered a "vanishingly rare" copy of the US Declaration of Independence.
Michael Scurr made the find at the Kew-based National Archives, where he volunteers, on what he described as a "boring, old Thursday morning".
The document is the sole example of its kind located outside the United States.
It's also one of just 11 surviving copies of a 1776 printing produced in Exeter, New Hampshire.
The historic text had remained hidden within the state's archive collection for centuries before Mr Scurr stumbled upon it.
Saul Nasse, the archives' chief executive, said: "This is an extraordinary discovery.
"It's a vanishingly rare surviving copy of the Declaration of Independence, found not in America, but here in the UK."
Mr Scurr, who has spent 11 years volunteering at the archive, came across the document in May whilst sorting through old captains' papers.
"I started reading a particular letter, which seemed to have an awful lot of enclosures, and I thought, 'oh, I'm going to be here all morning with this letter'," he told the Press Association.
"But I do have to read everything to make sure that I pick up everything that's important and relevant.
"After several of the enclosures, I came across this document, unfolded it, and it had in big letters on the top: Declaration.
"This is not something that I'd ever come across, and I thought, well, this is really exciting.
"So, I called across to my boss, Bruno, and said, I think you should come and have a look at this."
The document declared the independence of Britain's 13 American colonies from the Crown.
It was seized by the Royal Navy on Christmas Eve 1776 when British forces captured an American vessel.
Such copies were printed to spread word of independence rapidly throughout other British colonies.
Ships frequently carried these documents to drum up support for the revolutionary cause.
The Exeter printing is among the rarest versions of the Declaration in existence.
The National Archives already had three official printings of the Declaration of Independence prior to this discovery.
But none of those copies were anywhere near as rare as the one Mr Scurr unearthed.
Historians pored over his find for over a month before announcing it to the public.
He was also told to keep his discovery secret from fellow volunteers, friends and even his family while experts studied the document.
"A secret is something that needs to be kept... If it's not, it's not a secret, so I just kept quiet about it," he said.
Dr Graham Moore, a curator at the National Archives, told PA: "It's really significant for us because it's one of the few.
"We have a lot of copies, well, we have several copies of the Declaration of various different types.
"This one is so interesting to me because not only is it new to us and it's a rare one, there's only 11 now of this Exeter Declaration in the world."
Conservation work has been carried out to remove creases and prepare the document for public view.
It will soon be displayed at the Archive's Revolution 250 exhibition, which covers America's independence from 1763 to 1783.
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