A package containing heroin was found in an 80-year-old file at The National Archives in Britain.
The sealed package was discovered in October by a researcher who visited the Government's official archive in Kew, west London.
The researcher made the discovery while investigating a 1928 file from the British Consulate in Cairo, Egypt, detailing a criminal assault court case involving possession of narcotics.
The Class A drug was found when the researcher opened file FO 841/276, which revealed a pouch of 19 sachets containing the mysterious "off-white powder".
After the substance was analysed, conservation specialists confirmed it was less than a gram of heroin.
The file was temporarily removed from public access and the pouch and its contents were photographed before being handed over to police.
The file has since been replaced with a photo and the file is now back on public display.
Jeff James, director of operations and The National Archives said the discovery hints at more mysteries and "untold tales yet to be uncovered", hidden deep within the 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."
But this is not the first time an unusual item has been found. Researchers have also discovered a mummified rat, a red pyjama suit, and the death mask of Dr John Yonge.
The National Archives make records dating back more than 1,000 years available to the public.