A secret memo on redrawing the Irish border in the 1980s – including handing over west Belfast to the Republic – has been dismissed as "bonkers" and "madness".
Officials also discussed the possibility of establishing "a walled ghetto" in west Belfast, and a Berlin-style wall.
The 1984 proposals – which were swiftly scrapped – have been ridiculed by both unionists and nationalists.
Sinn Fein MLA Phil Flanagan, whose Fermanagh/South Tyrone constituency would have been reshaped by a new border, said it was another attempt at gerrymandering – the old unionist practice of artificially creating Protestant majorities by changing boundaries.
"It tells you the mindset which existed – and probably still exists – within the NIO," he said.
"Instead of trying to install a different border in Ireland, they would be far better served removing the one already there.
"They tried gerrymandering before. To do it again in the 1980s wouldn't have made any difference to the viability of a northern state."
DUP MP Jeffrey Donaldson branded them "silly nonsense".
"This kind of madness on the part of civil servants is just astounding," he said.
"The idea that you could repartition Northern Ireland, not only in terms of our land border with the Irish Republic but also internally, with Belfast divided into different legal jurisdictions similar to a mini Berlin, is just bonkers.
"I have no doubt it would have created massive problems in terms of further political instability and segregation.
"I'm glad wiser counsels prevailed and this type of silly nonsense never saw the light of day."
Proposals to redraw the border emerged in a paper prepared by NIO officials, based on a new partition put forward by Queen's University academic Paul Compton. It was among hundreds of previously secret papers released by the National Archives in London under the 30-year rule yesterday.
NIO officials revisited the frontier question in response to Dr Compton's "most respected analysis" of the Irish border.
Dr Compton had described the partition of 1920 as "necessary and justified", but "flawed by the messy way in which it was executed".
He suggested three options for repartition, including one ceding over half of the geographical area of Northern Ireland to the Republic, reducing its population to one million – 73.5% of it Protestant.
The NIO paper, discussing Dr Compton's suggestions, also referred to a possible partition of Belfast.
NIO officials discussed creating "a wedge-shaped area in west Belfast" running from Twinbrook to the Divis Flats. The briefing paper discusses "difficulties over the Belfast sector", adding that one solution, "a walled ghetto", "would entail physical as well as political difficulties".
Although the proposals were rejected, the fact they reached the Prime Minister's desk shows they were seriously considered.