Mr Peter Mandelson said at the Iveagh House ceremony:
The word is too often used and I have thought hard before using it now. But this is an historic day. Because today for the first time in 25 years, Northern Ireland's politicians have taken charge of Northern Ireland's affairs and put all the failure, the pain and the suffering behind them.
There will be difficulties ahead but I believe that the people of Northern Ireland - and indeed the people of these islands as a whole - will feel the benefits in their daily lives of the institutions being established today. Because their leaders have set out on the path to political stability and unbreakable peace and I believe they will not turn back.
And with this exchange of letters we open a new chapter in relations between Ireland, Northern Ireland and the rest of the United Kingdom - a relationship which is truly unique in its history and significance for both our countries.
For most of this century constitutional issues have divided the people, erected barriers and fuelled violence on this island.
But now, with the Good Friday agreement fully implemented, we can resolve those differences.
For the first time in 80 years all the people of this island have had a chance to express their view, to choose division or agreement.
Overwhelmingly they chose agreement, consent in peace over division, coercion and violence.
That choice is reflected today in the constitutional provisions that each government is bringing into force: the changes to Articles 2 and 3 of the Irish Constitution and the parallel British constitutional changes including the repeal of the Government of Ireland Act 1920.
This agreement and those changes have one common golden thread: the principle of consent.
Different aspirations will continue to be legitimately held about the future of Northern Ireland. But now there is agreement that the future of Northern Ireland can only be determined by and with the consent of the people of Northern Ireland.
The British-Irish Treaty which comes into force today sets in train a whole new architecture of institutional links throughout these islands: the North-South Ministerial Council, the British-Irish Council, the British-Irish Intergovernmental Conference and the six implementation bodies will all begin work today.
These are momentous steps.
They create a framework of institutions, with diverse responsibility but a single ethos.
Because the Good Friday agreement and George Mitchell's review of its workings have taught us that what we can do alone does not stand comparison with what we can achieve in partnership.
Partnership and consensus are at the heart of Northern Ireland's new government and they must inform all our dealings with it.
That is why these institutions are designed to enhance practical co-operation for the benefit of all the people of these islands.
That is why they bind our two governments and our diverse regions into a framework that allows us to share what we have in common and respect what makes us different. Because we work better when we work together.
And I believe that relations between our two governments are better than ever.
The close ties between the Taoiseach and the Prime Minister have made an incalculable contribution to progress in Northern Ireland. History will acknowledge them as the architects of this peace.
They built on foundations laid by their predecessors: John Bruton, Albert Reynolds and John Major have all, in some way, made today possible.
And I can say the same about mine, Mo Mowlam, Paddy Mayhew and Peter Brooke and all our officials whose patient toil is today finally paying off.
I pay tribute to David Andrews and Liz O'Donnell, whose insights and tireless efforts have been invaluable.
The two governments will need to work as closely together in the future as we have in the past. Everyone across these islands has been touched by conflict and everyone can share the rewards of peace.
Today we are taking another step towards the full implementation of the Good Friday agreement and putting in place a framework for real partnership and co-operation.
Another step towards a better future for all the people of these islands. A new partnership. An unbreakable peace. A friendship that will endure.