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Police are also examining the newly emerged material.
Many of the new files found were in a set of previously uncatalogued records called "Cabinet Secretary miscellaneous papers".
It included documents about three MPs - former home secretary Leon Brittan, Sir Peter Morrison and Sir William van Straubenzee - although the contents of those papers is not known.
The MI5 letter, written in 1986 by its then boss Sir Antony Duff to the then Cabinet Secretary Sir Robert Armstrong, followed an inquiry into an MP alleged to have a "penchant for small boys"
The letter concluded that the MP's denial was accepted and Sir Anthony wrote that "the risk of political embarrassment to the government is rather greater than the security danger".