A sealed package containing heroin was found in an 80-year-old Foreign Office file at the National Archives, its managers have said.
The Class A drug was filed with a document from the British Consulate in Cairo about a 1928 court case.
The off-white powder, discovered by a member of the public who asked to see the file, was sent for analysis.
And having been confirmed as heroin, the substance was handed over to the Metropolitan Police.
The pouch - which contained less than a gram of heroin in 19 sachets - has been replaced with a photo and the file is now back on public display.
National Archives director of operations Jeff James said the discovery hints at more mysteries to be found among the huge bank of archives.
"From time to time, unusual and occasionally valuable objects are unexpectedly discovered within our vast collection of 11 million records," he said. "However, finds of this nature are extremely rare."
The National Archives makes available to the public records dating back more than 1,000 years.
Alongside important historical records like MI5 files and the Domesday Book, researchers have also found unusual items, including a mummified rat and a red pyjama suit.