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 some 600,000 dossiers in 2011, when an historian came across a 45-year-old memo that referred to it.
The Foreign and Commonwealth Office (FCO) 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 newspaper to demand that in a free society 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 the University of 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," he told Reuters. "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 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 said.
"Because we haven't seen full listings of these documents, I don't want to speculate but the fact is that we do know these are very important materials," David Anderson, a professor of history at the University of Warwick, told Reuters.
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 which referred to the materials, the FCO also disclosed the existence of hundreds of thousands of other hidden files.
Some parts of Britain's 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," Margaret MacMillan, a professor of history at Oxford who is also part of the group, told Reuters.
"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 World War One), because I thought I'd seen everything that was there to be seen."
The scholars contrasted the secrecy over the documents with the revelations triggered by fugitive U.S. spy contractor Edward Snowden about the power of Britain's GCHQ eavesdropping agency, which is overseen by the foreign secretary.
"While the GCHQ story tells us that the government has wholly unexpected capacities to unearth information about its own citizens, the right of citizens to investigate UK foreign and colonial policy over the last 150 years and more is clearly being denied," the scholars said in the letter.
"Those of us who work on the history of some other countries are used to government obstruction when it comes to researching official papers, but the UK is supposed to be a free society. The writing of full and impartial accounts of the Cold War, Britain's colonial past, and other key subjects depends on access to all the available records."
When asked why the special collection had been held for so long, an FCO spokesman said: "These special collections have accumulated over time. They have been transferred into the FCO archive to ensure the records are preserved for potential selection and transfer to The National Archives.
"Historically the FCO has concentrated on the release of files in the departmental series and resources have not been available to review and prepare the special collections for release."