MI5 kept file on former PM Harold Wilson
Defence of the Realm, the first authorised history of MI5, says there were worries about his relationships with Eastern European businessmen.
His contact with KGB officers also raised concerns.
However, the book, serialised in the Times, dismisses long-standing claims of bugging and plots against him.
Mr Wilson, who died in 1995 aged 79, was the only serving prime minister to have a permanent Secret Service file, according to the book's author Cambridge professor Christopher Andrew.
MI5 opened the dossier in 1945 when Mr Wilson became an MP after communist civil servants suggested he had similar political sympathies.
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His contact with KGB officers also raised concerns.
However, the book, serialised in the Times, dismisses long-standing claims of bugging and plots against him.
Mr Wilson, who died in 1995 aged 79, was the only serving prime minister to have a permanent Secret Service file, according to the book's author Cambridge professor Christopher Andrew.
MI5 opened the dossier in 1945 when Mr Wilson became an MP after communist civil servants suggested he had similar political sympathies.