Support for the Ulster unionist cause was at one time a defining characteristic of Conservative politics. Since the 1970s, though, the unionist influence has been much weaker. One example illustrates the change. In 1912, at a huge unionist rally in Balmoral in south Belfast, the Conservative leader, Bonar Law, pledged his support for unionist resistance to Home Rule. Yet, at the same venue just over 80 years later, another Conservative leader, John Major, along with the Irish Taoiseach, unveiled the Framework Document, to the dismay of Ulster's unionists.

The unionist tradition, however, was never extinguished: far from it. Within the Conservative party since the 1970s there have always been those advocates of the Union ready to challenge the received wisdom and to offer an alternative way forward.

They included people such as Enoch Powell, Ian Gow, and John Biggs-Davison. Above all there was the doyen of the Tory unionists, the journalist TE (Peter) Utley, whose short book, Lessons of Ulster, remains the most brilliant account of the shortcomings and foibles of British politicians in the early years of the Troubles.

All of them were congenital high Tories. Their robust, non-sectarian unionism and belief in the integrity of the British nation were unshakeable. They were also bound together by their support for a policy that every British government had ruled out since the 1970s: known in shorthand as full "integration" with Great Britain.

Tory unionists had an instinctive distrust of grand initiatives, of the quest for a halfway house between unionism and nationalism, and of the attempts to apply a rationalist, essentially English liberal approach to Ulster politics. They bitterly regretted that the one leader they thought shared their unionist convictions, Margaret Thatcher, signed the Anglo-Irish Agreement.

By contrast, integration offered what Ian Gow described as "the least dangerous and most hopeful way forward". It was also consistent with the anti-devolution approach taken in the rest of the United Kingdom.

The influence of these Tories, and their ideas, remains profound. They are most clearly expressed in the leader columns of the Daily Telegraph and The Times or at meetings of the pressure group, Friends of the Union. Within the shadow cabinet Viscount Cranborne is the nearest that today's Tory unionists have to a spiritual leader.

It is with these Tory unionists that I have always sympathised since becoming involved in Ulster politics in the 1980s - first in the Conservative research department and then as a special adviser in the Northern Ireland Office. Those who know me will testify to my attachment to the purest doctrines of unionism and my innate scepticism - bordering at times on considerable unease - about many aspects of the peace process, especially the corrupting influence it has had on constitutional politics.

So how should Tory unionists react to the agreement reached at Stormont on Good Friday? It is to hope that it is carried by an overwhelming majority - on both sides of the Border - in the referendum. A number of reasons come to mind.

First, this is clearly as inclusive an agreement as is likely to occur. A large part of my scepticism was due to a disbelief that the talks process could work. As Peter Utley once put it, "The faith on which British policy has rested, that logically incompatible objects can be reconciled simply by submitting them to the healthy process of negotiation, has so far not been justified." Well, notwithstanding the difficulties that lie ahead, this process of negotiation has led to an agreement with which the majority of Ulster's political representatives can live, and if it lasts - and that is a big if - that fundamentally changes matters. Second, given the political forces ranged against them, I reluctantly believe that - with all its flaws - this is probably as good a deal as Ulster unionists are ever going to achieve. It is essentially a partitionist settlement.

If unionists reject this agreement they can be assured that when next they come to negotiate, nationalist demands for change will be stronger than ever. Unionists will be in a significantly weaker position. A repeat of the vote in 1974 that led to the political demise of Brian Faulkner might provide some with short-term comfort. For the long-term position of unionists - and the future of Ulster as a part of the United Kingdom - it would be a disaster.

Third, we need to consider the agreement in the wider context of changes that are taking place to the constitution throughout the United Kingdom. I am deeply fearful of the consequences of establishing devolved institutions in Edinburgh and Cardiff. The fact is Tony Blair is going to make sure that they happen. By the time of the next Conservative government the constitutional map of the entire United Kingdom will look decidedly different. At least the approach to Northern Ireland has a semblance of consistency with what is happening elsewhere.

There has always been a time for the Conservatives to resist change and there has always been a time for them to accommodate change. A number of elements of Friday's agreement bluntly stick in the throat, most notably the lack of real assurances on decommissioning. I also harbour large doubts about the workability of the institutions that are proposed. But we should be prepared to give it a chance. The greatest of all Irishmen, Edmund Burke, once said: "A man full of warm, speculative benevolence may wish his society otherwise constructed than he finds it, but a good patriot and a true politician always considers how he shall make the most of the existing materials of his country."

That to me describes a classic Tory approach to politics. I shall continue to believe that there were times when integration was the right policy. But we cannot turn back the clock. The task of all Conservatives and unionists is now surely to make the most of this agreement and ensure that it works to preserve that which we most hold dear - the Union itself.

Jonathan Caine was special adviser to the Secretary of State for Northern Ireland 1991-'95