Steve Aiken has formally taken over as leader of the Ulster Unionist Party (UUP).
The former Royal Navy commander was appointed unopposed as UUP leader at a meeting of the party.
He was the only candidate who put his name forward to replace Robin Swann, who announced he was stepping down as leader in September.
On the issue of Brexit, Mr Aiken said the UUP did not support the prime minister's deal.
He said he was in favour of remaining in the EU in order to protect the union.
"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 said.
He added: "We joined the European Union together as a United Kingdom, we either leave the European Union as a United Kingdom.
"If we can't do that - and that deal's not on the table because the only thing on the table is now Boris Johnson's deal - we're going to have to remain in the EU to be able to achieve that.
"That is the only way we can keep the United Kingdom together."
In a speech to party members, Mr Aiken said the UUP needed to reform its structures and "significantly boost" membership.
He also called on the UUP to carry a "message of hope" about Northern Ireland's future.
In his first speech as leader on Saturday, Mr Aiken described himself as a "proud son of a senior trade union official" who grew up in County Antrim "steeped in working-class unionism".
He referred to his nickname "Submarine Steve", explaining that he had been "privileged to command two nuclear submarines" during his 31-year career in the Royal Navy.
Mr Aiken told party members that his "political awakening" happened when he heard former UUP leader Mike Nesbitt talk about the need for a "new Northern Ireland".
He said this was a "place that needed to realise that we had more in common than anything that kept driving us apart, and it was Mike's vision that attracted me to our party - a message of hope for Northern Ireland, not fear".
Mr Aiken faced a difficult few weeks in the lead up to his ratification as leader when he had to backtrack on his announcement that the UUP would field candidates in all 18 constituencies for the forthcoming general election.
The UUP also called in police to investigate threats against staff after its headquarters received phone calls about its refusal to form an electoral pact with the DUP.
Mr Aiken is taking the reins at UUP headquarters after a long period of disappointing election results.
The party once dominated the political landscape in Northern Ireland and was influential at Westminster through its strong links with the Conservatives.
In the 2017 general election, the UUP lost both its remaining MPs, leaving it with no representation in the Commons for only the second time in more than a century.
In the Northern Ireland Assembly election earlier that year, it won 10 seats and was beaten into fourth place by the SDLP who won 12.