Britain considered ethnic cleansing Northern Ireland, reveal documents issued to the Public Record Office under the 30-year rule.

In 1972 Edward Heath's Conservative government drew up secret proposals to expel up to 500,000 Catholics from Northern Ireland to create an "avowedly sectarian statelet".

"Great resistance could be expected from many of those who should move," warned the Whitehall planners urging ministers to sanction "ruthless force" if the scheme was given the go-ahead.

The year was a grim one in the province - then, as now, under direct rule from London - it had begun with "Bloody Sunday", the gunning down of 13 civil rights protesters by British soldiers, heralding a death toll of 500 in 12 months.

Plans to create a predominantly Protestant state were given to the then prime minister, Heath, on July 23, just two days after another "Bloody" 1972 day - when 11 people died and 130 were injured in a Belfast bombing spree following the collapse of a brief IRA ceasefire.

The top secret document, signed by the cabinet secretary, Sir Burke Trend, marked the UK's growing desperation at a situation many believed approached civil war.

"We are on the point of losing control of events unless we take very severe action indeed," he warned.

A zero-sum option of expulsion of nationalists was to be preceded by a massive military crackdown under the Cabinet Office plan - in a last bid to find a security solution to the troubles.

Troop numbers were to be doubled from 20 battalions to 47 with the aim to "swamp with troops all extremist strongholds including 'no-go' areas or any other area where there is a risk of inter-sectarian strife, to achieve complete domination and demoralisation of extremists of both sides impartially".

If the military crackdown failed the plan envisaged a "political solution" for Northern Ireland - dividing the province into nationalist and unionist areas.

Without revealing who commissioned the proposals, the document contemplates the forced removals of 200,000 to 300,000 Catholics into areas which would be handed over to the Irish Republic.

Another 200,000 Protestants would be moved to districts that would remain part of the UK.

"In this connexion we have, as requested, specifically discussed the feasibility and implications of a redefinition of the border with the Republic and compulsory transfers of population within Northern Ireland between the Six Counties and the Republic," the document states.

"About one third of the population of Northern Ireland would be on the move. Such a massive movement would not be peacefully accomplished; great resistance could be expected from many of those who should move."

And the civil servants behind the plan urged the use of "ruthless force" to see the ethnic cleansing through.

"Unless the government were prepared to be completely ruthless in the use of force, the chances of imposing a settlement consisting of a new partition together with some compulsory transfers of population would be negligible."

But throughout the document officials make it clear that they were "extremely doubtful" that such a policy could work.

"We do not believe that the government would be able to obtain the support of public opinion in Great Britain for the drastic actions that we consider in this paper, or to avoid major controversy in parliament unless the situation in Northern Ireland was manifestly such as to warrant the declaration of a 'great emergency'," the officials warn.

"Any faint hope of success must be set against the implications of a course which would demonstrate to the world that HMG was unable to bring about the peaceful solution of problems save by expelling large numbers of its own citizens and doing so on a religious basis."