Following restoration works, the copy will be displayed in the archives’ exhibition on the path to American independence, which opened last month.
Saul Nassé, chief executive of The National Archives, praised the find as “an extraordinary discovery”.
He added: “It’s a vanishingly rare surviving copy of the Declaration of Independence, found not in America, but here in the UK.”
The document was seized by the Royal Navy on Christmas Eve 1776 when the HMS Raisonable captured an American ship, the Dalton, off the coast of Portugal following a seven-hour pursuit.
Dr Graham Moore from The National Archives said the discovery is “one of the rarest forms of the Declaration we know about”, adding that it was not meant to be preserved due to the intention to distribute it quickly.
“After the original printing on 4 July, the news of the Declaration is travelling fast around North America and its being reprinted as it reaches each successive colony,” he told BBC News.
“The copy we have is one of only 11 surviving from the first ones printed in New Hampshire.”
