LONDON—British researchers have announced the discovery of a rare original copy of America’s Declaration of Independence—just in time for the Fourth of July.

Katrina McClintock, a spokeswoman at the National Archives, said Thursday that a researcher accidentally discovered the “Dunlap print,” named after a printer, several months ago. The find was announced only after it could be properly catalogued.

Edward Hampshire, the National Archives’ specialist in colonial materials, said the find was “incredibly exciting.”

“It is likely that only around 200 of these were ever printed, so uncovering a new one nearly 250 years later is extremely rare, especially one in such good condition,” he said.

The last Dunlap print to be found was sold at an auction for $8.14 million in 2000.

National Archives officials said they weren’t sure how the Declaration of Independence copy ended up there.