New documentary evidence detailing a British plan to guarantee a united Ireland if the government agreed to enter the second World War on the British side is released at the Public Record Office in London today.

The British plan, drawn up in June 1940, envisaged an immediate declaration accepting "the principle" of a united Ireland, the establishment of a Joint Defence Council and a joint body to deal with the constitutional detail of unity and the possibility of merging the administrations North and South. In return, the then Taoiseach, Eamon de Valera, was asked to abandon the policy of neutrality.

But Ireland could remain "non-belligerent" if the government invited British ships into Irish ports and British troops and aircraft were allowed access to Irish territory to secure the country against a German invasion while protecting Britain's western flank.

In a Dominions Office file from June 27th, 1940, released as part of the open government initiative, the Secretary of State for the Colonial Office, Malcolm MacDonald, told the Taoiseach over lunch that if the plan was accepted "a united Ireland would come into actual being within a comparatively short period of time".

The meeting was the culmination of discussions between the two men over 10 days in Dublin when de Valera had outlined his opposition to abandoning neutrality and renewed his calls for Britain to supply Ireland with arms to defend itself against German attack.

De Valera eventually rejected the plan on the grounds that Dublin could not be sure London would fulfil its guarantee of a united Ireland, his belief that Britain would lose the war and the fear of dissension within the Fianna Fail party.

The then Northern Ireland Prime Minister, James Craig, was incensed when he was told about the plan for a united Ireland.

At the Dublin meeting, which was also attended by the Minister of Supplies, Sean Lemass, and the Minister for External Affairs, Frank Aiken, Lemass said there was no guarantee in the plan that a united Ireland would be established immediately. Indeed, de Valera had made the same point during a meeting with MacDonald on June 26th when he described the offer as a "deferred payment" for entering the war.

But MacDonald countered by claiming that while London "would be glad if a united Ireland could come into being at once", there were practical difficulties that made such a proposal impossible. The three governments would need to consider at length a new constitution. Merely extending the Irish Constitution to include Northern Ireland would not work either, he told de Valera.

Dublin also raised concerns about Northern Ireland's willingness to unite. MacDonald told de Valera London would not "coerce" Northern Ireland into an agreement and "would not and could not march troops into the six counties to force a policy upon their government". Nevertheless, the present circumstances "offered a very good chance of such an agreement being reached".

Aiken then raised the possibility that British security could still be guaranteed if Northern Ireland was informed that Ireland would remain neutral. But MacDonald dismissed Aiken's proposal, pointing out that Northern Ireland's role in the war was "most valuable to us".

MacDonald then asked to speak not as a representative of the British government, but in a private capacity as an individual "whose sympathies were on the side of the establishment of a united Ireland".

He told de Valera and his colleagues that they faced a stark choice. "If the leaders of Eire now stayed out of the war, and perhaps contributed to German strength by doing so, whilst the people of Northern Ireland and of the United Kingdom were joined in the supreme struggle against the Nazis, then none of us in Britain would be very concerned to create a united Ireland afterwards."

A German invasion of Ireland, MacDonald said, would "extinguish Irish freedom" during the war, but if Ireland's defences were increased it would make a German attack much less likely.

But de Valera argued that national unity would be broken if British troops were stationed on Irish soil and Ireland's neutrality would be prejudiced, exposing the country to a greater risk of German attack.