MR David Trimble has strongly denounced economic arguments which seek to promote all Ireland institutions.
In a speech in Dublin to the Institute of Directors in Ireland, the leader of the Ulster Unionist Party rejected the idea of a Belfast Dublin economic corridor, a joint tourism authority for the whole island, and joint representation in negotiations with Brussels.
"Any attempt to fool the voters into supporting a political aim through the use of false economic arguments is reprehensible, and will be resisted," he said.
"Unionists are suspicious when economic arguments are used by both British and Irish governments to support nationalist demands for cross Border institutions with executive powers."
Northern Ireland and the Republic had much in common, and their close proximity and natural and cultural ties made them natural trading partners, he said. The Ulster Unionist Party strongly supported all mutually beneficial trade and economic collaboration.
"What we oppose is not any useful economic contact, but cross Border institutions which are devised for political rather than economic purposes as part of an agenda to foist either political unity or joint authority on the unwilling, electors of Northern Ireland.
He specifically rejected the idea of a Belfast Dublin economic corridor and the suggestion that a single tourism board should serve the whole of Ireland.
"Even the suggestion [of a corridor] raises the hackles of all those living outside the corridor, and any concentration of jobs in this area would conflict with wider locational aims in both jurisdictions," he said.
The most pressing need for both states was to improve sea borne communications with Britain.
As for tourism, the need of the Northern Irish tourism industry was to market Northern Ireland. Tourists already came to the island of Ireland. "The problem has been ... to get them to our part of the island."
"More important" he added, "is the role which tourism plays in defining and fostering national identity. It is important to unionists that an all Ireland, essentially Gaelic image of shamrocks, Irish music and Guinness is not slowly imposed on Northern Ireland, which has a range of different traditions."
However, he described as "perhaps most subversive of all to our interests" the suggestion that the two states should be jointly represented in negotiations with the European Union.
"Although there are clear North South similarities in agriculture, Northern Ireland is an integral part of the United Kingdom with all that implies for taxation, public spending, interest rates and regulatory procedures as well as, of course, for national identity."
He made no reference to the BSE crisis, apart from a jocular mention of the success of the Government in keeping Northern cattle from travelling south, compared with certain traffic in the other direction. But he acknowledged past difficulties with milk quotas.
"These are quickly rectified through representations through London," he added.
"There are obvious economic dangers in loosening ties with the UK in order to strengthen them with Dublin or Brussels. In any case the economic arguments are largely irrelevant when national identity is at stake.
"While we welcome more trade and co-operation with the Republic, I must however confess that we do not see it as necessarily more important than co-operation with Great Britain, the rest of Europe, or elsewhere."
Asked if he was rejecting the idea of greater economic co-operation, he said it was a question of market size. The island of Ireland had a population of about five million.
"Already we are in a 55 million market in the UK, and the 365 million market of the European Union. It is a question of what is the larger market. Otherwise you are getting back to the old Sinn Fein economic ideas which were rejected by the Irish government in the 1950s."
Comparing the Republic and Northern Ireland, he said both had achieved significant growth in recent years.
In the case of Northern Ireland this was mainly achieved by indigenous companies. Small locally owned companies now contributed a quarter of total output from Northern Ireland industry.
He said that, while per capita GDP in the North was similar to the Republic's, living standards were higher. This was because of lower taxation rates and higher per capita expenditure on public services in the North.
"The combination of low taxes and high public expenditure of course leaves a financial gap. Most of this is paid for by the UK exchequer as part of the normal functioning of the fiscal system in the UK," he said.
He said that there was a common misconception that cross Border trade was abnormally low. In fact, at the beginning of the decade Northern Ireland producers sold three times as much to each person in the Republic as they did to each person in Britain.
The removal of Articles 2 and 3 of the Constitution would facilitate greater economic co-operation, he said.