DUP leader Sir Jeffrey Donaldson has insisted the Union has not been weakened by the death of the Queen.
Sir Jeffrey said times of change bring "uncertainty" but he was confident the new King Charles III will continue to "build the unity of our United Kingdom".
He said Sinn Féin's prominent role in events during the monarch's visit to Northern Ireland was a "sign of progress" and a "mark of how far we've come".
Read more: Sinn Féin's 'evolving' relationship with the British monarchy
Sir Jeffrey and other Stormont party leaders met with King Charles on Tuesday at Hillsborough Castle, the royal residence in Northern Ireland, to express their condolences on the death of his mother.
The DUP leader was asked on BBC's Good Morning Ulster whether the Union is weaker as a result of the Queen's death.
Sir Jeffrey responded: "I don't think that is the case, and of course there has been an outpouring of emotion across Northern Ireland and Her Majesty was someone who was very dear to very many people in Northern Ireland, and for many it feels almost like they have lost someone very close to them, a part of their family.
"She was a constant in the lives of so many, and most of us in Northern Ireland have never known any other head of state.
"So of course at a time of change it brings uncertainty, but I have every confidence that our new King will continue in the same vein as his mother.
"That he will want to promote and support reconciliation as she did, that he will continue as our head of state to build the unity of our United Kingdom."
Sinn Féin vice-president Michelle O'Neill was among those who met the King, while the Stormont Assembly Speaker, Sinn Féin's Alex Maskey, delivered a message of condolence on behalf of the people of Northern Ireland.
Sir Jeffrey welcomed how all the main parties paid their respects, adding: "I think that is a sign of progress in Northern Ireland that there is a recognition that King Charles III is the head of state of the United Kingdom."
The DUP leader was asked by the King about his party's position on Brexit's Northern Ireland Protocol, which has angered unionists for introducing new trade barriers in the Irish Sea.
The trading arrangements have sparked a power-sharing crisis at Stormont, with the DUP blocking the restoration of the Stormont Executive in protest.
Asked about his conversations with King Charles, Sir Jeffrey said: "Well of course the protocol is a challenge that we face at the moment. It is a major issue for Northern Ireland. It impacts on our relationship with the rest of the United Kingdom.
"Of course it would be right that His Majesty takes an interest in matters that disrupt the unity of the United Kingdom."
The Lagan Valley MP said it was clear the King "very much cherishes Northern Ireland as part of the United Kingdom".
"And he was very clear that he wants to help and he asked each of us if we felt there were ways in which he as head of state could help in Northern Ireland - he wanted to hear that," he added.
In his speech on Tuesday, Mr Maskey reflected on the Queen's "positive leadership", adding: "Let us be honest with ourselves enough to recognise that too often that leadership has been lacking when it has been most required."
Sir Jeffrey said he did not believe those remarks were aimed at his own party.
He said: "I don't think so. I think that was a comment that was meant to capture the reality that all of us have at times struggled to provide the leadership that Northern Ireland has needed.
"But look, as far as my own position is concerned, I know that I have the support of the people who want to see the issues resolved and resolved properly, and that was very clear when I stepped out of Hillsborough Castle yesterday and walked down the street, and the cheers that I received from the crowds.
"And I'm not making a point out of that, I'm simply saying that people understand what I'm trying to achieve, which is political stability and progress in Northern Ireland that is based on firm and sound foundations."
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