The Belfast Agreement, if endorsed by the people, will create a new basis for the relationships within Northern Ireland, between North and South, and between Ireland and Britain. Many important goals are reflected in the document: the achievement of reconciliation, tolerance and mutual trust; the protection and vindication of human rights; and a commitment to partnership, equality and mutual respect.

However, the cornerstone on which the agreement is built is its common ownership by the people of this island: any change to the system of government will in future require the consent of the governed, because the future system of government will belong to the people.

So, the British government undertakes to introduce fresh legislation which repeals the Government of Ireland Act, 1920, and supersedes all previous laws governing the status of Northern Ireland. The new Act will recognise that Northern Ireland remains within the United Kingdom only by and with the consent of its people. The Act will bind the British government to give effect to any future declared wish of a majority that Northern Ireland should cease to be part of the United Kingdom and become part of a united Ireland. Effectively, in Northern Ireland, uniquely within the United Kingdom, the constitutional framework will recognise that it is neither Parliament nor the Crown which is sovereign, but the people.

For our part, we are asked to make corresponding changes to the Irish Constitution. The new wording for Article 3 recognises that a united Ireland shall be brought about only by peaceful means with the consent of a majority of the people, democratically expressed, in both jurisdictions in the island.

Last week in these pages the Taoiseach pointed out and I agree with him, that, for the first time, a precise mechanism has been defined and accepted by the British government by which a united Ireland can be put in place by the consent of Irish people and that alone. I want to concentrate here on the phrase from the Taoiseach's article which I have highlighted.

The new Article 3 does not simply set out a preferred means of unifying the country: it stipulates that securing consent within both jurisdictions is the sole means capable of achieving unity. This has significant consequences.

If any group were to resort to violence in the cause of unity, it would of course be acting contrary to the declared wishes of the people, as expressed in referendums in both parts of Ireland. Suppose, however, that such a group achieved its ultimate objective and that the British government announced its intention to withdraw from Northern Ireland. Not only would the British be in breach of their treaty obligations under the British-Irish Agreement, but the Irish government would still be disabled, both by international law and, more importantly, by the terms of the Constitution, from taking any non-consensual steps to attain unity.

If an Irish government, in such circumstances, either attempted a unilateral annexation of Northern Ireland or, by agreement with the UK, accepted a handover of some or all of the territory of Northern Ireland, it would be acting unconstitutionally and its actions would be of no effect in Irish law.

We are being asked to make a binding commitment that Irish unity cannot validly be brought about in future except by consent. If we approve this proposal, as I am confident we will, continuation of the armed struggle will become not just undemocratic but futile, since it will be incapable of achieving its objective. The insurmountable obstacle will not be British colonialism or unionist obduracy but the giving of legal effect to the expressed will of the people, in the fundamental law of this State. Without overthrowing that law and the State itself, violence can no longer in any circumstances achieve the objective of its perpetrators.

After May 22nd, safeguarding the principle of consent will be the new constitutional imperative.

For those who reject what they describe as the "unionist veto", the choice is a stark one: to act in contravention of the consent principle will amount to seeking the destruction of the institutions of this State as well as those of Northern Ireland. The goal of physical-force nationalists - an enforced British withdrawal and subsequent Irish assimilation of the occupied counties - will become unattainable because, while it may be possible to bomb the British administration out of Northern Ireland, it is not possible to bomb the 3 1/2 million citizens of this State into a united Ireland against their will.

Since its foundation in 1970, the SDLP has argued that the unification of Ireland could be brought about only by consent. That principle was formally adopted by the Irish government three years later when, in Paragraph 5 of the 1973 Sunningdale Agreement, the Irish government fully accepted and solemnly declared that there could be no change in the status of Northern Ireland until a majority of the people of Northern Ireland desired a change in that status. The consent principle was restated in Article 1 of the 1985 Anglo-Irish Agreement and reaffirmed in Paragraph 5 of the 1993 Joint Declaration. It is a matter of great pride for me that my party was involved in government when these three milestone agreements were produced.

Despite the opinion to the contrary of the IRA leadership, I am satisfied that the decision taken North and South of the Border on the Good Friday agreement will indeed amount to an exercise of national self-determination. When the Irish nation endorses the principle of consent and confers on it constitutional status, dismissive references to the unionist veto will become redundant. Unity by consent will be the stated aspiration of the nation; unity by consent alone will be the guarantee made on behalf of the nation to all people living on this island.

Ruairi Quinn is the leader of the Labour Party