Former Prime Minister Harold Wilson thought he was being spied on by the Russians while he was on the Isles of Scilly, reveal files just made public.
Some rogue MI5 officers thought Wilson was a Soviet agent
The Labour leader feared that Russian ships disguised as trawlers were trying to eavesdrop on him at his holiday home off Cornwall in 1974.
Ministry of Defence officials sought to reassure No 10 there was no evidence the Soviets were spying.
The papers have been released by the National Archives at Kew, west London.
It is unclear what sparked Wilson's concerns during the summer of 1974.
But he was warned about the dangers of discussing sensitive information on the "walkie-talkie" he used during bird-watching expeditions on the islands.
Wilson returned to power in February 1974 and his final years in Downing Street were increasingly dominated by his fears that the security service, MI5, was plotting against him.
It later emerged that some rogue MI5 officers - including Spycatcher author Peter Wright - were convinced that Wilson was a Soviet agent.
But the prime minister's main concern at the time was about a Russian threat.
On his return to London from the islands, Wilson told his private secretary, Lord Bridges to ask the MoD whether they had noticed any "unusual activity" by Soviet vessels off the islands during his stay.
Senior MoD official Bill Mumford said one trawler which the Soviets routinely used for intelligence-gathering operations had sailed within 30 miles of the islands, which was not unusual.
"The vessel's patrol when the prime minister was in the Scilly Islands was not, therefore, a new departure," Mumford informed Bridges.
"In short, we are not aware of any indications to suggest that Soviet vessels attempted to intercept communications with the prime minister."
The Cabinet Office learnt of Wilson's concerns and warned Bridges that it was possible for Wilson's conversations on his "walkie talkie" - which were not encoded - to be intercepted.
Bridges told Wilson: "Although the range of the transmitter receiver is theoretically short, there is no doubt that its signals are a greater risk when transmitted from the island to the mainland and Whitehall.
"I expect you realise this, but draw the comment to your attention."
Wilson won four general elections as Labour leader, two between 1964 and 1970 and two in 1974 - the first of which failed to gain a majority. He resigned as prime minister in March 1976.
His press secretary, Joe Haines, said he was surprised by the latest revelations.
"But as part of a pattern of paranoia I don't think I'm particularly surprised now," he told BBC Radio 4's Today programme.
"He gradually began to suspect everybody. He suspected MI5, he feared a military coup, he thought the Soviets or anybody else might be spying on him and it got worse and worse I'm afraid."
Mr Haines said in retrospect he thought the fears were part of the decline in Wilson's mental powers.
He recalled how the prime minister had shown him wires sticking out of a wall behind a painting in Downing Street and suggested he was being bugged.
"It looked a lot to me like a lazy electrician had taken down a wall light and covered it up with a picture," added Mr Haines.