Calls for special status for Northern Ireland post-Brexit are an attempt to create a "united Ireland by the back door", Ulster Unionist leader RobinSwann has said.
Mr Swann also dismissed Sinn Féin demands for an Irish border poll as "nonsense".
Speaking at the launch of the UUP's manifesto in Templepatrick, Mr Swann said that if Northern Ireland was granted special designated status after its withdrawal from the EU, it would lead to the break-up of the United Kingdom.
Sinn Féin, the SDLP and Alliance parties have been calling for special status for the region.
However, Mr Swann said special status "brings all sorts of connotations that leaves us outside the UK".
"It sets us adrift to what it means to be part of the union," he added.
Mr Swann said that the EU referendum "is done" and insisted that instead of "re-arguing that argument" he would push for the best deal possible for Northern Ireland.
He stressed that there can be no border in the middle of the Irish Sea and no passport checks for citizens of Northern Ireland arriving in Cairnryan or Heathrow.
He said the party's key Brexit priorities include no hard border, or internal borders within the UK, maintenance of the Common Travel Area and assurances for those currently in receipt of EU funds.
When asked what he thought about Sinn Féin calls for an Irish border poll he responded: "Nonsense."
He claimed that a border poll would sectarianise every future election in the region.
He said: "No-one has anything to fear from being part of the United Kingdom but we all have a lot to lose from leaving it.
"The Union, that is the United Kingdom of Great Britain and Northern Ireland, not only is best for the present but it's best for future generations."
"I believe the Ulster Unionist Party is best placed to promote the message that a strong, confident and prosperous United Kingdom, of which Northern Ireland remains an integral part, is the best outcome for all of us," he added.
Mr Swann also said he wants Northern Ireland's health trusts to be merged into a single body in an overhaul of the healthcare system.
The party's manifesto says that to reform the region's "broken" healthcare system the UUP would merge the five health and social care trusts and put in place an independent chief executive of the NHS in Northern Ireland.
The party also vows to secure the "best possible deal" for the region in the event of a "bungled Brexit deal".
"Despite having potentially the most to lose from a bungled Brexit deal, Northern Ireland remains by far the least prepared of any UK region," the manifesto says.
It adds: "This issue is so important to all our futures that we need the best deal, not just any deal, or worse, no deal."
Legacy issues are mentioned with the UUP vowing to "vehemently oppose efforts to rewrite the reality of the past and tackle unfair aspersions cast against legitimate actions of our security forces".
A single education system is also one the party's key issues in the eight-page document.
