Eminent British scholars have challenged the government to release a cache of secret files dating back almost 400 years that they say could spur a reappraisal of some contentious episodes of British colonial history and the cold war.
The foreign ministry only publicly admitted the existence of the so-called "special collection" of 600,000 dossiers in 2011, when an historian came across a 45-year-old memo that referred to it. Among the documents are reports on the slave trade dating back to 1662.
The Foreign and Commonwealth Office says it did not have the resources to sort through the files, despite laws requiring disclosure, but that it is now reviewing them to prioritise material of greatest public interest for release.
But a group of 27 scholars from the British Academy used a letter to The Guardian to demand all available records be released to ensure a full account of Britain's colonial past or the espionage mysteries of the cold war.
"The government must release these files," said Richard Evans, a professor of history at Cambridge who is an expert in German history and one of the signatories to the letter.
"A full and objective account of Britain's colonial past, involvement in the cold war and many other important historical topics will only be possible when they are freely available. Of course, I can't say how they might change our view of the past since I don't know what's in them."
From the realpolitik of Britain's imperial past and the African slave trade to the betrayals of KGB double agents during the cold war, the true historical worth of the documents is unknown, historians say.
"Because we haven't seen full listings of these documents, I don't want to speculate, but the fact is we do know these are very important materials," said David Anderson, a history professor at the University of Warwick.
Held in six boxes, for example, could be important material related to slavery. Simply entitled "Slave trade reports", the boxes contain 18 files from the period 1662 to 1873.
Three years ago, Anderson was an expert witness in a court case brought against the British government by 5,200 Kenyans who say they were tortured by Britain's colonial forces during the 1950s Mau Mau insurgency.
Forced to make public documents on Kenya after Anderson found a 45-year-old Whitehall memo that referred to the materials, the Foreign and Commonwealth Office also disclosed the existence of hundreds of thousands of other hidden files.
Some history could need to be revised in the light of the new files, Anderson said.
The unexplained secrecy of such a large cache of documents has fuelled suspicions that they could contain surprises for those Britons with a benevolent view of British foreign policy.
In Anderson's view, standout items in the trove could include nuggets to do with the cold war, perhaps shedding new light on the activities of Guy Burgess and Donald Maclean, two spies acting for the Soviet Union.
"The trouble is we don't know what's there," said Margaret MacMillan, a professor of history at Oxford. "Are there things tucked away in there which would actually make me change my view on, for example, the British role at the Paris peace conference [after the first world war], because I thought I'd seen everything that was there to be seen."