Details of secret US-UK 'spying pact' released
A previously top secret intelligence-sharing agreement between Britain and America is being released to the public for the first time.
Until a few years ago, even the existence of the agreement was not acknowledged by the two governments.
Signed in 1946, it remains the basis for the sharing of intercepted communications between the countries.
Some of the material shared on the Soviet Union in the 1940s is also being released by the National Archives.
During World War II, Britain and America had co-operated closely on so called "signals intelligence" - intercepted communications.
When the war came to an end, the two sides decided to institutionalise that co-operation and establish it in the new context of the emerging Cold War with the Soviet Union.
'Truly global coverage'
As well as revealing the deal itself, the National Archive files lay bare the negotiations which led to its signing on 6 March 1946, and the follow-up agreements throughout the 1950s that were needed to make it operate in practice.
"It's the nuts and bolts of how it works," said Dr Edward Hampshire, principal records specialist at the National Archives.
Details of the agreement are being released simultaneously on both sides of the Atlantic following separate Freedom of Information requests to British and American governments.
"These are the foundational documents of the largest intelligence alliance in history - one that has had truly global coverage and shaped events decade after decade since the Second World War," said Richard Aldrich, author of a recently published history of GCHQ (Government Communications Headquarters).
Thousands of files reveal private conversations and correspondence between Soviet citizens, military personnel, Communist Party officials and religious leaders in the period from 1946 to 1949.
These US and UK intercepts give a flavour of life within the Soviet Union at the time, ranging from local crime statistics to details of economic woe and the banning of local folk songs.
There are also insights into the bureaucracy of the Soviet state, and in one file there are reports of a colonel in the Ministry of Internal Affairs being subjected to a court of inquiry.
The reports talk of a "stormy meeting" and one person is quoted as saying: "Of the Moscow representatives, nothing remains but a wet spot."
Dr Hampshire said: "For any historians of the Soviet Union and the tail end of the Stalin era, it gives you a fascinating view of what was going on.
"It gives you an indication of the extent to which the centre in Moscow was imposing itself really on a granular level on the whole Soviet Union."
The intelligence alliance still operates today, and since the incorporation of Australia, Canada and New Zealand, it has been known as the "five eyes".
Earlier this year, in an interview with the BBC, director of GCHQ Iain Lobban explained how it works now.
"It is based on sharing where it is possible to do so," he said.
"So where we can save each other work, where we can bring analytic effort together, where we can actually produce reporting for the greater good we will do so."