The chain of British title to Northern Ireland, which began in 1800, has been transformed by the Belfast Agreement.

The issue has been subjected to little public scrutiny, however, because of Government fears of creating difficulties for the Ulster Unionist leader, Mr David Trimble, in the forthcoming Northern referendum. The changes to Articles 2 and 3 of the Irish Constitution and the deletion of the territorial claim to the North have, at the same time, come in for detailed examination.

To make any assessment as to whether the Union is strengthened or weakened by the Belfast Agreement, it is necessary, first of all, to spell out the changes in the constitutional relationship between Northern Ireland and the United Kingdom now proposed.

Corresponding to the Irish commitment to change Articles 2 and 3, the British government has undertaken to introduce legislation "relating to the constitutional status of Northern Ireland". Both governments have agreed that the British legislation and the amendments to the Irish Constitution must be introduced before the Belfast Agreement comes into force.

The contents of the British legislation are laid out in Annex A of the "Constitutional Issues" section of the Belfast Agreement. Clause 1 and the Schedule (see panel) enshrine the principle of consent into British constitutional law. Northern Ireland in its entirety remains part of the United Kingdom "and shall not cease to be so" without the consent of a majority of the people there. If a majority decides that Northern Ireland should form part of a united Ireland, the Secretary of State will give effect to that wish. Political and legal sources see Clause 2, however, as the key constitutional provision in the Belfast Agreement. It states: "The Government of Ireland Act 1920 is repealed; and this Act shall have effect notwithstanding any other previous enactment". (The Act referred to is the new British legislation). This clause goes to the core of Britain's statutory chain of title to the sovereignty of both parts of Ireland, North and South.

The Government of Ireland Act 1920, in Section 1 (1) provided for two parliaments, one for Northern Ireland and one for the South. Section 1 (2) defined Northern Ireland: "For the purposes of this Act, Northern Ireland shall consist of the parliamentary counties of Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone, and the parliamentary boroughs of Belfast and Londonderry".

Section 1 (1) was repealed in part by the Northern Ireland Constitution Act, 1973. Section 1 (2) was confirmed by Agreement, on December 3rd, 1925, and the Ireland (Confirmation of Agreement) Act, 1925. The key part of the Government of Ireland Act which is now being repealed is Section 75 stating that "the supreme authority of the Parliament of the United Kingdom shall remain unaffected and undiminished over all persons, matters, and things in Ireland and every part thereof".

The replacement of Section 75 with the principle of consent is revolutionary in British constitutional terms. Northern Ireland will have a unique status within the United Kingdom because, for the first time, the people, rather than the Parliament, will be sovereign and supreme.

A key question is raised, however, by the further wording of Clause 2 stating that the new British Act "shall have effect notwithstanding any other previous enactment".

Since the Act of Union 1800 is the legal basis for incorporating Northern Ireland within the UK, are sections of it superseded by the Belfast Agreement?

The Act of Union was passed by equivalent, but not identical, statutes in Westminster and Ireland. In archaic language it began: "That it be the first Article of the Union of the Kingdoms of Great Britain and Ireland, that the said Kingdoms of Great Britain and Ireland shall, upon the first Day of January which shall be in the Year of Our Lord One thousand eight hundred and one, and for ever after, be united into one Kingdom, by the Name of The United Kingdom of Great Britain and Ireland".

Many of the Act's provisions were virtually repealed by the legislation passed consequent to the establishment of the Irish Free State and the Parliament of Northern Ireland but, according to the authoritative Halsbury's statutes, "the Act remains the statutory warrant for the continued incorporation of Northern Ireland with the United Kingdom".

Notwithstanding the Act of Union, however, there is no doubt that the principle of consent in the Belfast Agreement replaces the three constitutional guarantees given by Britain to Northern Ireland this century.

The first was in the Government of Ireland Act, 1920. The second and third were in the Ireland Act, 1949, and the Northern Ireland Constitution Act, 1973.

The latter guarantees, almost similarly worded, stated: "Northern Ireland remains part of Her Majesty's dominions and of the UK, and it is hereby affirmed that in no event will Northern Ireland or any part of it cease to be part of Her Majesty's dominions and of the United Kingdom without the consent of the majority of the people of Northern Ireland voting in a poll held for the purposes of this section."

So what is the state of the Union in the wake of the Belfast Agreement? The Ulster Unionists say it has been strengthened. Nationalists and republicans believe it has been weakened.

Irrespective of their interpretations, however, the most significant constitutional development in the Belfast Agreement relates to the status of Northern Ireland. The British government is transferring sovereignty to the people of Northern Ireland. But it is also giving the people of the Republic a possible say in the status of Northern Ireland. The Belfast Agreement will form part of the Irish Constitution. Its provisions can only be changed by reference to the people - North and South - in a further referendum.