Secret surveillance of Britain’s ­notorious double agent, Kim Philby, made public for the first time in archived documents, reveals how keenly the Security Service wanted to confirm or disprove early suspicions of his high-level treachery.

In daily bulletins submitted to MI5 in November 1951, undercover operatives describe how Philby, codenamed Peach, moved about London.

They said he gave “no outward sign of being either nervous or on the alert, but your well trained man should not do so; every movement is natural – again as it should be”.

The notes, from official ­“watchers” who were tailing Philby and bugging his phone, raise a key question about how the arch-traitor eventually escaped justice: did the British establishment deliberately protect him, or simply hope to avoid a public scandal? Mark Dunton, of the National Archives, believes the ­documents, which go on display in the ­exhibition, MI5: Official Secrets, next week at Kew, west London, shed light on one of the most shady ­periods of British ­espionage.

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