A new excuse for burying bad news? Cambridge professor seeks assurances from Foreign Office over declassification of its archives

Some 600,000 documents that shed light on Britain's recent past may be released only slowly, or not at all, for lack of staff

The exact contents of the archive remain unclear, but an initial inventory suggests some intriguing possibilities

The leading historian appointed to oversee the release of a vast cache of secret government documents has warned that sufficient resources are required to ensure that the papers, which could revolutionise understanding of aspects of British history, are not withheld for decades.

Professor Tony Badger, master of Clare College at Cambridge University, said he was seeking assurances from the Foreign and Commonwealth Office (FCO) that it would employ enough experts to declassify its vast archive of 600,000 files within an acceptable time frame.

His warning came as a group of some of the world's most eminent historians revealed they intend to mount unprecedented legal action against the Government over concerns that the FCO is dragging its feet in tackling the cache, which dates back almost 400 years.

The historians, who allege that the FCO has broken two separate laws by failing to declare its huge holding, believe the so-called Special Collections have the potential to spark a reappraisal of events from the African slave trade to Cold War espionage. Amid fears that the process of disclosure is being slowed in order to preserve state secrets, an application for a judicial review of the process could shortly follow.

Among the most significant papers in the archive are 4,000 files relating the experience of Britons who suffered Nazi persecution, including victims of the Holocaust.

Professor Badger, appointed in December as the independent reviewer of the Special Collections, said the process of assessing the cache was a "massive undertaking" undertaken in good faith. He said he was concerned that the FCO should provide enough staff to meet its obligations to declassify documents of historical importance. Based on the recent disclosure of another previously secret FCO archive of 20,000 files detailing colonial-era atrocities, which took 30 months to complete, it would take 75 years to clear up the Special Collections.

Professor Badger said: "I will have to get assurances that there is enough capacity to work through this material. It is a massive undertaking and it is important to establish whether there will be enough resources.

"I can understand why there is suspicion among my fellow historians. But one of the best proofs of the commitment to openness is that the Foreign Office don't want to be embarrassed again, as it has been in the past."

Officials were forced to admit the existence of the 20,000 files in the colonial-era Migrated Archive, during litigation in 2011 to secure compensation for Kenyan victims of torture suffered under the British during the 1950s Mau Mau rebellion.

The files, which helped to lead to a £20m damages settlement with survivors last year, came to light only after a historian working on the case found a 45-year-old Whitehall memo mentioning the material. Even then, the FCO failed to disclose that the Migrated Archive was just a small part of the much bigger Special Collections, which were originally estimated to hold 1.2 million files, but which officials now insist has 600,000 dossiers.

Since its existence was quietly made public in 2012, the FCO has twice had to seek a special 12-month dispensation from the Justice Secretary, Chris Grayling, while it decides how much of the archive to declassify. The department announced in December that it will spend six years assessing 60,000 files from the archive, kept at Hanslope Park, near Milton Keynes and classified as "high priority".

Academics believe that that undertaking is insufficient and offers no assurances about what proportion of documents will be released or when. It is understood that the FCO has six full-time staff sifting through the 600,000 files for transfer to the National Archives.

Professor David Anderson, of Warwick University, who discovered the memo that led to the disclosure of the Special Collections, said the FCO was likely to have been in breach of the Public Records Act 1958 and the Freedom of Information Act 2000 by failing to disclose the existence of the files. Along with other leading historians, including Professor Richard Drayton of King's College London, Professor Sir Richard Evans of Cambridge University and Professor Margaret MacMillan of Oxford University, Professor Anderson is contemplating a lawsuit to force the FCO into full and more timely disclosure.

He told The Independent on Sunday: "We are hoping that we will get the response we seek from the Government, that they are doing everything that is reasonably practicable to release this material. But our worry is that the Government's statements so far are so incredibly vague that they are self-evidently designed to bury this issue. "

The case is being led by Dan Leader of law firm Leigh Day, which acted for the Kenyan claimants. "It would potentially be a judicial review if the Government's proposals do not comply with statutory obligations," he said.

Professor Badger said the "eclectic" nature of the documents suggested that many were files which officials had struggled to classify and then became lost in a bureaucratic backlog.

But others are concerned that previous practices, under which documents were transferred to already "open" files, meaning that they were unlikely to be noticed, cast doubt on the commitment to shine a bright light on all aspects of Britain's past.

The Foreign Office insists that it is committed to releasing the archives "in line with our obligations and with maximum transparency".

Dodgy dossiers

The exact contents of the archive remain unclear, but an initial inventory suggests some intriguing possibilities. Among the files are documents entitled "Nazi war criminal in Britain", "Australian war crimes", "Minister and massacres: papers relating to the repatriation of Cossacks" and photographs of the scene of the suicide of Rudolf Hess.

There are 18 files from the period 1662 to 1873 containing reports on the slave trade, which may cast light on the continuation of slave sales by British merchants after the practice was banned in Britain and its dominions in 1807.

A further 4.5 metres of files relate to KGB spies Guy Burgess and Donald Maclean, including the fallout in Whitehall over who to blame for their 1951 escape to Moscow.