In the 1960s and 1970s, a section of the British security apparatus, mostly within MI5, believed that Labour Party leader Harold Wilson was a Soviet spy.
They never proved their case (presumeably because there was no case in the first place) although the disruption and damage this belief caused was considerable. For anyone with even a passing interest in the security services, Robin Ramsay and Stephen Dorrill's 1992 book 'Smear: Wilson and The Secret State' is a must read.
It was not however, the last word. Proof of this comes via the spy writer Nigel West (actually the former Conservative MP for Torbay Rupert Allason) who penned 'When the KGB sought seats in the Commons' in The Daily Telegraph of 30 June. West/Allason wrote:
"Concern about Eastern European intelligence personnel ensaring MPs dates back to suspicions in 1962 that an Opposition leader had been assassinated to make way for a Soviet agent to take his place, and the best candidate was Harold Wilson, who succeeded Hugh Gaitskell following his mysterious death. Wilson became the subject of a highly secret MI5 investigation codenamed "Wheatsheaf", that remained unresolved."
Surely, in that it proved nothing, that meant the case was resolved? And Harold Wilson was not a Soviet asset?
*Hat tip to KM in Yorkshire for passing the West article down to me.