Mikhail Gorbachev launched an obscene outburst at Neil Kinnock when the then Labour Leader raised the issue of human rights and the treatment of dissidents, it has emerged.
The incident happened a private dinner in 1984 when Kinnock brought up the plight of imprisoned Natan Sharansky.
"This provoked an intemperate outburst of obscenities against 'turds' and spies like Sharansky, who was in prison 'and that is where he will stay'," the British interpreter who accompanied him throughout the six-day visit, KA Bishop, wrote.
"He warned, with appropriate gestures, that Britain would get it 'right in the teeth' in a 'merciless' denunciation of its human rights crimes if that is the game that it wanted to play.
"With his usual adroitness, however, Gorbachev collected himself and and told Mr Kinnock that he had never spoken so undiplomatically on the subject to anyone else during the visit, and that such frankness was possible only between people who refer to each other as 'comrade'!"
The encounter left a "chill impression" on the interpreter.
"We had glimpsed beneath the surface a man conscious of power and ready if need be to exploit it ruthlessly," he wrote.
"The combination of cleverness, modern-mindedness, Slav nationalism, energy, charm, self-assurance and single-mindedness would make him at worst a formidable adversary and at best an interlocutor to be treated with the utmost respect and circumspection."
Previously unseen documents also reveal how Gorbachev turned up unannounced at Downing Street only for his surprise diplomatic initiative to fall flat when he found Prime Minister Margaret Thatcher was not in, according to newly-released government files.
In December 1984, the future Soviet leader made his first visit to western Europe and made an immediate impact as an energetic reformer with the potential finally to transform the sclerotic communist state.
Following their official talks at Chequers, a clearly impressed Mrs Thatcher famously declared that he was a man she could "do business with".
Mr Gorbachev's success seems to have gone to his head, inspiring him to make an impromptu follow-up visit to No 10, according to the British interpreter who accompanied him throughout the six-day visit, KA Bishop.
According to Mr Bishop's account of the visit, contained in files released by the National Archives in Kew, west London, Mr Gorbachev was undaunted that his effort came to nothing.
"His unscheduled and unannounced visit to a 10 Downing Street empty of its principal resident was probably less a caprice and more an example of his confidence and decisiveness (of which there were several other instances) and his apparent conviction that problems exist to be solved," he wrote.
Like all who met him during his stay, Mr Bishop was struck by the contrast between the charismatic Russian and the dour, stony-faced old men of the Kremlin with whom the West had become accustomed to dealing with.
"My close observation of him as his interpreter throughout his official programme leaves me in no doubt that Gorbachev was eminently the right man for the job," he wrote in a vivid pen portrait which was sent to Mrs Thatcher.
"There was about his movements and his utterances an unaffected, self-assured and un-self-conscious air of competence and confidence.
"While showing on occasion that he could trade if necessary in the language of the dialectic, he kept his remarks throughout the week notably free of the familiar Marxist/Leninist jargon, bombast, 'preachiness' or cliches.
"A roguish twinkle was never far from his eye (he even once winked at me over his shoulder as I interpreted a neat parry of his to one of the Prime Minister's verbal thrusts)."