A rare surviving copy of the Declaration of Independence has been discovered at The National Archives in Kew, the only known example of its kind outside the US.

The document was uncovered by a volunteer in February while cataloguing the papers of Royal Navy captains from the American Revolutionary War.

It is one of 11 copies printed in Exeter, New Hampshire, in July 1776 to spread news of American independence through the colonies before it was seized by British forces.

Volunteer Michael Scurr recalled feeling butterflies in his stomach after he opened up the paper and realised what it was.

"I called over to my boss and said, 'I think you need to come and have a look at this'," he told BBC News.

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 HMS Raisonable captured an American ship, the Dalton, off the coast of Portugal after 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 because it was intended to be distributed 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."

The captured ship was then brought back to Britain and its papers seized, including the privateer's commission, printed instructions from the Continental Congress, and the Declaration itself.

Moore says the Declaration was found folded among the letters of Captain Thomas Fitzherbert and was brought to Plymouth in January 1777 before being moved to Whitehall in London.

The document was listed without distinction by the Royal Navy captain, who described it as "another paper" at the time, and it had remained hidden in the state's archive for centuries.

Moore said the treasure is the only known copy of the Declaration taken by military action.

The rare copy has undergone conservation works to stabilise its paper and repair a slight tear, making it safe for handling, study and future display.

It will go on display as part of Revolution 250: America's Independence Story, 1763-1783 at The National Archives.

The National Archives already holds three of the original official copies of the Declaration of Independence printed by John Dunlap in Philadelphia on 4 July 1776.

Around 200 copies are likely to have been printed on the night, of which only 26 are known to have survived until today.