Nineteen sachets of the class A powder was hidden inside an old Foreign Office file from 1928.

It related to a drugs and assault case handled by the British Consulate in Cairo, Egypt. T

It was tested by government  and passed onto the police.

Jeff James, operations chief at the centre, which holds 11million records, told the Sun that finds of this nature are extremely rare.

“This discovery hints at more mysteries and untold tales yet to be uncovered, hidden deep within the archives,” James said.

Other strange items found at the archives include the death mask of Dr John Yonge, Dean of York in the 16th century.

Another find was a red pyjama suit worn by a man arrested in 1932 at a private ballroom.

He was charged with being intimate with 32 other men while wearing women’s clothing.

James said: “From time to time unusual and occasionally valuable objects are unexpectedly discovered.”