The text, famous for its rallying cry for “life, liberty and the pursuit of happiness”, had been listed in ​18th-century records as simply “another document” but in May a volunteer at Britain’s National Archives ​took a closer look.

“Unearthing and handling such a significant historical document has been thrilling, ⁠particularly in this important anniversary year,” said Michael Scurr, the volunteer who found it while working on ​a cataloguing project.

Declaration found outside US

News of the discovery was published on Friday, the day before ​the US celebrates its semiquincentennial Declaration of Independence from Britain, when millions of Americans will enjoy cookouts and display the Stars and Stripes outside their homes.

Back in 1776, July 4 was when the declaration was adopted by Congress, and as the revolutionary mood ​swept through the American colonies, supportive printers rushed to reproduce it and share as widely as they ​could the new country’s founding document.

The one found in London was printed in Exeter, New Hampshire, in mid-July 1776 and ‌is ⁠the 11th surviving copy of the so-called “Exeter Declarations”, and the first to be found outside the United States.

Eleazer Johnson, captain of the Dalton ship, picked up a copy some months later that same year before he set sail across the Atlantic on a mission to try to seize British vessels, possibly hoping the ​text would inspire his crew ​to fight for the ⁠new country.

But on December 24, 1776, off the coast of Portugal, the Royal Navy captured the Dalton and brought it and its contents back to Plymouth, ​southwest England.

The National Archives said the document is the only known copy of ​the declaration taken ⁠by military action.

Under the red tape of late 18th-century England, all British captains had to present a captured ship’s documents to the authorities in order to claim their share of the prize.

Given the British seized 3,600 ⁠ships ​during the American Revolutionary War, the National Archives offers a vast ​and fertile hunting ground for historians.

“Thanks to the bureaucratic processes of war … we can present an unusually rich backstory that most surviving ​declarations do not have,” Graham Moore, curator at the National Archives, said.