Former British prime minister Margaret Thatcher's government privately signalled that it would not stand in the way of a united Ireland a year after sweeping to power.
State files released for the first time show the reputedly hardline Conservative administration told Dublin it had a greater interest in Northern Ireland than London.
But the then Secretary of State Humphrey Atkins confided in Foreign Affairs Minister Brian Lenihan that "there would be an explosion" if it emerged they were making plans towards reunification.
"One step would have to be taken at a time," he said, according to government notes of a meeting between the two on April 15, 1980.
"There was 'no way' he could go round promoting Irish unity. This was simply not possible. That was not to say however that it was something that the British Government would stand in the way of - but it could not promote it."
Mr Atkins insisted that persuasion was needed to remove genuine Protestant fears and apprehensions.
The previously classified notes of the meeting in Dublin show Mr Atkins - considered by many an uncompromising Tory - advised then Taoiseach Charles Haughey on the apparent British position.
"The Secretary of State indicated that he had said to the Taoiseach that the Irish Government's interest in Northern Ireland was greater than any other party except of course the people of Northern Ireland," the notes reveal.
A year later Mrs Thatcher memorably remarked that "Northern Ireland is as British as Finchley."
The documents released from the Taoiseach's office, under the 30-year rule, show the Irish Government was already pushing for a three-strand resolution focusing on North/South and British/Irish dimensions, as well as cross-community relations within the North. The model would eventually form the basis of the Good Friday Agreement 18 years later.