Outgoing UUP leader Doug Beattie always sought to act in the best interests of Northern Ireland, Secretary of State Hillary Benn has said.
Mr Beattie quit on Monday, citing “irreconcilable differences” with party officers.
Senior political figures have paid tribute to his leadership and constructive approach to relations with other parties.
Mr Benn thanked Mr Beattie for his “commendable leadership”.
In a statement, Mr Benn said: “It is clear that he has always sought to act in NI’s best interests and I know he will continue to do so.
“I look forward to working closely with his successor.”
Sinn Féin’s vice president Michelle O’Neill said the party will take a “constructive approach” with the new leader of the UUP.
Ms O’Neill said: “I want to wish Doug Beattie and his family well as he announces today that he will step down as leader of the UUP.
“We have worked constructively since he became leader of his party, including during the restoration of the Executive and institutions at the beginning of this year.”
The first minister added: “I will continue this constructive approach with the new leader of the UUP when they take up post.
“Sinn Féin will continue to work together with all parties in the Executive and Assembly to ensure we deliver for all.”
Tánaiste Micheál Martin said Mr Beattie led his party through “numerous challenges”.
Mr Martin said: “I’ve always appreciated his emphasis on making politics work & the constructive relationship we built.
“I know he will continue to lead by example whatever the future holds.
“A strong relationship between this government and political leaders in Northern Ireland is vital – I look forward to building the same ties with Doug’s successor.”
SDLP leader Colum Eastwood paid tribute to Mr Beattie’s dedication to serving the people of the north.
He said: “Doug and I have different politics but I’ve always admired his candour and commitment to working together in the interests of the people we represent.
“I know this won’t be the end of his political contribution but hope he enjoys some more time to spend with his loving family.”
Alliance deputy leader Dr Stephen Farry wished Mr Beattie well for the future, adding: “He will no doubt continue to make a significant contribution to politics and public life.”
Former Tánaiste Simon Coveney said he was “sorry” to see Mr Beattie step down.
Mr Coveney, who was the Republic’s foreign affairs minister during Brexit negotiations between the UK and the EU, said: “I always found Doug to be fair, tough when needed, reasonable and positive.
“Of course we didn’t agree on everything, but Doug was someone I trusted and respected as a strong unionist leader.”