A year after she was nearly killed in the IRA's 1984 Brighton bomb, the then prime minister expressed dismay at Catholic opposition to British rule when they could follow the example of ancestors who were evicted from Ulster at the barrel of a Cromwellian gun in the 17th century.

Lady Thatcher's extraordinary solution to the Troubles has been disclosed by her advisers at the time of the negotiations on the 1985 Anglo-Irish agreement.

Sir David Goodall, then a diplomat who was one of the most senior British officials negotiating with the Irish government, told a BBC four-part documentary, Endgame in Ireland, that Lady Thatcher made the "outrageous" proposal during a late night conversation at Chequers.

"She said, if the northern [Catholic] population want to be in the south, well why don't they move over there? After all, there was a big movement of population in Ireland, wasn't there?

"Nobody could think what it was. So finally I said, are you talking about Cromwell, prime minister? She said, that's right, Cromwell."

Her interest in him is likely to turn her into an even greater hate figure among nationalists, who have never forgiven her for mishandling the 1981 republican hunger strikes. Catholics were slaughtered in their tens of thousands in the 1640s and 1650s by Cromwell's forces. Virtually all Catholic landowners were hounded out of Ulster.

Lady Thatcher's "outrageous" plan did not stop at reviving the memory of Cromwell.

Sir Charles Powell, then her private secretary, told the programme that she also called for Northern Ireland's border with the republic to be redrawn.

"She thought that if we had a straight line border, not one with all those kinks and wiggles in it, it would be easier to defend," he said.

The zigzag border is notoriously difficult to patrol. But Lord Armstrong of Ilminster, then cabinet secretary, told Lady Thatcher of the folly of her idea.

"It wasn't as simple as that because the nationalist communities were not all in one place, not all in Fermanagh and Tyrone and South Armagh and so on," he told the programme.

"There were many in Belfast, and the idea of partition in Belfast or moving large numbers of population didn't seem to be very attractive."

However, she would not abandon her idea and called for a "security zone" on both sides of the border to help the British army and the RUC to chase IRA terrorists who used to slip over the border after attacks in the north. This was rejected out of hand by the Irish government.

The border remained intact, and Lady Thatcher signed the Anglo-Irish agreement in November 1985, giving Dublin a consultative role in Northern Ireland in return for greater cross-border cooperation on security.

Her view of Ireland may have been tainted because her political ally Airey Neave was killed by a car bomb planted by the Irish National Liberation Army two months before her 1979 election victory. In 1990 Ian Gow, her parliamentary aide, was killed by an IRA car bomb.