A breath of fresh air appears to have swept through MI5, the notoriously secretive British security service, over the ten years since the Open Government Initiative began - particularly where history is concerned. The secret service is beginning to open its archives, reports John Crossland after a visit to the Public Record Office.

Not only did its director general, Sir Stephen Lander, himself a historian, give a keynote speech at a recent conference of intelligence historians and researchers at the Public Record Office, but he announced that the service is interviewing veterans of its second-world-war and cold-war operations to ensure their vital collective memories are built into a departmental archive of 400,000 files.

The initiative is similar to the American approach of using living testimony to enliven and deepen understanding of dry, often fragmentary files. It also shows how MI5 is responding to the demands of a developing field. But while the MI5 initiative was welcomed by historians, there was frustration that other parts of the secret service appear to have remained untouched by the new openness, although one positive step has been the staged release of files from the Special Operations Executive, the service dedicated to sabotage and covert operations to destabilise the enemy.

Richard Aldrich, professor of politics at Nottingham University and cold war intelligence expert, vented a general dissatisfaction with the continuing Special Intelligence Service (MI6) policy of almost total closure, a policy endorsed by former foreign secretary Robin Cook in formulating the Freedom of Information Act. He said: "Historians and the SIS are ultimately at loggerheads. There is a conflict of interest. Historians will always want to get at archives in time to interview participants."

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He was also critical of academic historians, saying too many treated material trickling into the PRO from parallel secret agencies to MI6 as "a free lunch". It was, he claimed, all too often pre-digested pap, which made the researcher who fed on it flabby. He urged his listeners to follow the example of their US colleagues who, aided by a more open legislative framework, were resorting to the techniques of investigative journalism.

The uncovering of the arms for Iraq scandal was one example. Historians of the Office of Strategic Service (the American equivalent of the SOE) were able to unearth the truth thanks to freedom of information legislation that allowed the operational details to be revealed before the veterans involved died and their "hard discs were wiped".

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Aldrich added that, while MI6 was "hostile to history", the national signals interception and interpretation centre, GCHQ, is still relatively unknown territory as far as the immediate post-second-world-war period is concerned, despite releasing a sizeable quantity of raw intelligence decrypts from the war. He understands that even now a GCHQ officer is posted in the Foreign Office with the specific task of ensuring that signals intelligence files routed there go back to the Cheltenham HQ and are not mixed in with files that researchers are allowed to see at the PRO.

Despite the problems in gaining access to source material, there has been much recent interest in signals intelligence. At the PRO conference, Antony Best, of the London School of Economics, revealed the number of British and American agents spying for the Japanese before the second world war. The precursors of GCHQ started intercepting Japanese signals traffic as early as 1916 and Best discovered raw intelligence transcripts for every year thereafter that blew the cover of spies such as squadron leader Frederick Rutland. It is only in the past five years or so that this kind of information has emerged.

But much remains to be uncovered, including early signals interception and spying in the Middle Eastern campaigns of the first world war and British intelligence spying on neutrals in the second. Eunan O'Halpin, senior research fellow at Trinity College, Dublin, told the PRO conference that he had evidence of a British-Finnish deal on sharing radio intelligence in 1939 that resulted in a flow of 800 messages to Britain in one day alone. When Finland allied itself with Germany, however, "the fruits of the liaison were to be appreciated not by our code breakers at Bletchley Park, but by Berlin".

In the spirit of freedom of information, Gill Bennett, chief historian at the Foreign and Commonwealth Office, invited historians at the conference to write in with their requests and said that, in addition to forthcoming German intelligence material, diplomatic papers "from before VJ Day might be considered for release, but nothing with on-going sensitivity".

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A GCHQ sequence of top-secret correspondence between Churchill and Roosevelt is also making its way through the system. And for those frustrated by MI6's unbending approach, Bennett added that just because material was officially closed it did not mean that it did not find its way into the public domain, released into other departments' records with individual researchers sometimes being tipped off.

The Public Record Office is showing the exhibition, Shaken Not Stirred, a History of Espionage from the 17th Century to the Cold War. Telephone 020 8392 5202/5323.