The National Archives is best known for dusty files rather than Class A drug powders, so it came as a shock when a researcher found heroin there.
In October a scholar visiting the Government's official archive in Kew, west London, asked for a 1928 Foreign Office file from the British Consulate in Cairo, Egypt, detailing a criminal assault court case involving possession of narcotics.
Upon opening file FO 841/276, the researcher was intrigued to find a pouch containing 19 sachets with a mysterious "off-white powder" inside.
After analysing the substance with a spectrometer, conservation specialists in the archives' Collection Care Laboratory conclusively confirmed it was under one gram of heroin.
The file was temporarily removed from public access, and the pouch and its contents photographed before being handed over to the Metropolitan Police.
Jeff James, director of operations and services at the National Archives, said: "From time to time unusual and occasionally valuable objects are unexpectedly discovered within our vast collection of 11 million records, however finds of this nature are extremely rare.
"Whilst it highlights the diversity of our collection and its relevance to our nation's history, this discovery also hints to more mysteries and untold tales yet to be uncovered, hidden deep within the archives."
It is not the first time a strange substance has been found at the archives. In 2009 samples of an unknown powder were found hidden among hundreds of sealed letters in a collection of records from an 18th-century Dutch East India Company ship.
Other unusual objects kept at the National Archives include:
:: A mummified rat discovered among fragments of parchment solidified into a lump.
:: The death mask of Dr John Yonge, Dean of York and the 34th Master of the Rolls charged with the custody of the Court of Chancery's archive from 1508 to 1516.
:: A red pyjama suit believed to have been worn by a man arrested in 1932 at a private ballroom and charged with conspiring to corrupt public morals by dancing and being intimate with 32 other men whilst wearing women's clothing and make-up.
The mummified rat is of particular significance because it was used as evidence in an 1836 inquiry into the poor management of the record service held by Parliament's Select Committee of the Record Commission.
This led to the creation of the Public Record Office, which is one of the organisations that make up the National Archives today.