The UK's chief legal adviser emerged from anaesthetic after cardiac surgery and shouted that then-DUP leader Ian Paisley needed to be killed for progress to be achieved in Northern Ireland, Irish State papers report.
The extraordinary incident involving Attorney-General Sir Michael Havers is detailed in previously classified Irish diplomatic documents.
The revelation came during a bizarre 1987 lunch between Richard Ryan, an Irish Embassy official in London, and Ian Gow, a Tory MP later murdered by the IRA. Mr Gow was highly thought-of in Dublin, and had very close UUP links.
The briefing document, dated January 13, 1987 and marked 'confidential - by courier service', detailed a conversation between both men about Northern Ireland. It has been made public as part of the 1988 State papers release in Dublin.
During the lunch, the duo were approached by Sir Michael.
Mr Ryan wrote to his superiors: "At this point we were joined by the Attorney-General Sir Michael Havers, who stayed for an hour [and that ended the private conversation with Gow].
"Havers said that when he came out of the anaesthetic after his heart operation [his wife and the surgeons were there] he immediately shouted: 'We must kill Paisley!' He cannot explain it, he said, but 'wouldn't it confirm everything Paisley accuses us of?'"
Sir Michael's comment came after Mr Gow had expressed disillusionment over lack of progress in Northern Ireland.
Mr Ryan described Mr Gow as "depressed" and said he was "getting absolutely nowhere with his unionist clients".
Mr Gow said the current unionist leadership was "hopeless" and he felt that UUP leader James Molyneaux had "slipped off the hook" in an attempt to get him to meet with the Taoiseach, Charles Haughey.
Mr Ryan wrote that Mr Gow felt the UUP leader had "chickened out" because of his concern about being re-elected as MP.
He added: "He, Gow, if given such - or any - marching orders by his constituency 'would tell them to go to hell'. [But] he has no spark of hope left."
Mr Gow was murdered by the IRA in a car bomb outside his home in East Sussex in 1990.