The new Ulster Unionist leader has accused Boris Johnson of proposing a “sell-out” Brexit deal.
The Prime Minister and his DUP allies made “false promises” over the UK’s departure from the EU, Steve AIken added.
No unionist voted for England, Scotland and Wales to leave but Northern Ireland to stay in the bloc, he added.
Mr Aiken said: “It would be a momentous time to assume the leadership of any political party, but now, for Unionism, for those that are pro-union, and those that just believe in Northern Ireland, it has never been more important for the Ulster Unionist Party to be ready to go out and fight for what is right for our nation.”
The former submariner succeeds Robin Swann and will be aiming to win back representation at Westminster for a party which was once the mainstay of unionism but has been eclipsed in the polls by the DUP.
Mr Aiken outlined a vision of inclusive and progressive unionism.
He was forced into an election U-turn before taking up the role.
His party did not field a candidate in opposition to the DUP in North Belfast to maximise unionist representation at Westminster.
The incoming leader earlier ruled out voting pacts.
The UUP voted, on balance, to remain, during the EU referendum.
Its new chief said: “If the Conservatives deal goes through Northern Ireland will, well and truly, be a place apart.
“We will be separated from our largest market, with differing legal systems, tax regimes and held accountable by special and joint EU committees.”
He told a party meeting in Templepatrick, Co Antrim: “There are many unionists, and indeed many in this room, who will have voted to leave but I am pretty sure you didn’t vote for England, Scotland and Wales to leave while Northern Ireland stays.
“We have a stark choice, but for us, the union of our whole UK must come before anything else.”
He said the country either left the EU as one or remained as one.
He added: “With the future of our very Union being challenged by Boris Johnson’s sell-out deal, with the DUP’s ineptitude in creating a border in the Irish Sea, coupled with a level of venality that would, in any normal political society, see them banished into obscurity… we can be so much more and we have to be.”