Tributes are flooding in from across Northern Ireland following the death of Queen Elizabeth II.
The Queen died at Balmoral on Thursday afternoon. She was 96.
The King said it was a "moment of great sadness for me and all members of my family".
"We mourn profoundly the passing of a cherished sovereign and a much-loved mother," Charles said.
"I know her loss will be deeply felt throughout the country, the Realms and the Commonwealth and by countless people around the world.
"During this period of mourning and change, my family and I will be comforted and sustained by our knowledge of the respect and deep affection in which the Queen was so widely held."
Sinn Fein vice president Michelle O'Neill - and Northern Ireland's First Minister in waiting, said she had learned of Queen Elizabeth's passing with 'deep regret'.
In a statement she said: "It’s with deep regret that I learned of the passing of Queen Elizabeth II. The British people will miss the leadership she gave as monarch.
"I would like to offer my sincere sympathies and condolences to her children, and wider family as they come to terms with their grief I wish to especially acknowledge the profound sorrow of our neighbours from within the unionist community who will feel her loss deeply.
"Personally, I am grateful for Queen Elizabeth’s significant contribution and determined efforts to advancing peace and reconciliation between our two islands.
"Throughout the peace process she led by example in building relationships with those of us who are Irish, and who share a different political allegiance and aspirations to herself and her Government.
"Having met Queen Elizabeth on a number of occasions alongside my colleague, the late Martin McGuinness, I appreciated both her warmth and courtesy."
Sir Jeffrey Donaldson, DUP leader, offered his tribute: "Her Majesty The Queen has been a steadfast and unshakeable Head of State for the United Kingdom and the Commonwealth.
"Her gracious approach has been a constant throughout our lives.
"In 1952 during her first Christmas broadcast Queen Elizabeth II asked the nation to pray 'that God may give me wisdom and strength to carry out the solemn promises I shall be making, and that I may faithfully serve Him and you, all the days of my life'.
"Today we mourn Her Majesty’s death, but we do so with tremendous honour for one who served God and her people faithfully.
"I remember with fondness her speech in 2011 where she again referred to her Christian convictions and reminded us that forgiveness lies at the heart of her faith and that 'it can reconcile divided communities'.
"Her Majesty led by example in Northern Ireland and reached out the hand of friendship to help with the reconciliation process. We are duty bound to build on that foundation.
"The Royal visit to the Republic of Ireland was ground-breaking and the warmth with which Her Majesty was received demonstrated that she was revered and respected far beyond the United Kingdom.
"Her visits to my constituency in Royal Hillsborough invoke precious memories for the residents and for all of us and I know her death will be felt acutely in that village.
"Today we pray for the Royal Family as they mourn the passing of a mother, grand-mother and great-grand-mother."
Alliance Leader Naomi Long expressed her condolences.
"In the first instance, my thoughts and prayers go out to the entire Royal Family at this sad time. They are mourning a much-loved mother, grandmother and great-grandmother," said Mrs Long.
"But while they have lost a matriarch, the entire country and indeed Commonwealth has lost a person who has been a figure-head and leader for 70 years. Her loss will be keenly felt by many.
"Her Majesty Queen Elizabeth II is one of the figures who came to define the 20th Century, as well as a large part of the 21st due not only to her longevity but her commitment and dedication to the country and Commonwealth.
"The majority of us have only ever known her in the role of sovereign and it will be incredibly strange to not see her there carrying out her duties.
"Throughout her reign, the Queen worked tirelessly in service to the country and Commonwealth, receiving praise from all quarters for her selfless dedication to the role.
"That is only one of the aspects of her life which will remain as her lasting legacy, and I hope it brings her family, and indeed all those grieving, some comfort at this sad time."
Ulster Unionist leader Doug Beattie MLA said it was "one of the saddest days the United Kingdom has ever known."
He said: "The Queen was a regular visitor to Northern Ireland in good times and bad, and even during the decades of violence she was determined to come here to see her people and, in the process, provide comfort and reassurance.
"Her State visit to the Republic of Ireland in May 2011 went a long way to healing old wounds and normalising relations between two close neighbours and indicated very clearly her great wisdom and sound instincts."
Traditional Unionist Voice (TUV) leader Jim Allister said it was "a very sad day in the life of our nation."
He said: "To the nation and almost everyone alive she has been an intrinsic part of our lives.
"Her steady leadership and devotion to duty have hallmarked a reign of unparalleled success. Adjusting to life without Her Majesty will be a major national challenge.
"With a new King everything will seem different, yet the abiding constancy of the monarchy and its centrality to our constitutional arrangements remains.
"In now conveying sympathy to the Royal Family I thank God for the reign of Queen Elizabeth II. No nation could have asked for more from their monarch and no nation could have received more."
Meanwhile, SDLP leader Colum Eastwood also posted a tribute on social media.
He tweeted: "Saddened to hear of the passing of Queen Elizabeth this evening.
"I know her loss will be felt acutely by people across our island. Even the blessing of a long life does not make the burden of saying goodbye any lighter.
"Ar dheis Dé go raibh a hanam."
Northern Ireland Secretary of State, Chris Heaton-Harris, also paid tribute.
He said: "Throughout her reign, Her Majesty has been the matriarch and pillar of our country and the Commonwealth. She has always provided wise counsel, admirably and selflessly, fulfilling a lifetime commitment to public service.
“Her Majesty dedicated herself to our family of nations, no more so was this apparent than in Northern Ireland, We will always be greatly indebted for her efforts to promote reconciliation and the fostering of better community relations, ensuring peace and stability.
“I want to offer my sincere condolences to The King and the entire Royal Family on their loss. The thoughts of the people of Northern Ireland, like the rest of the United Kingdom and around the world, are with them at this time.”
Irish Foreign Affairs Minister Simon Coveney offered his condolences saying the news came as a "great personal sadness".
"Queen Elizabeth’s sad passing marks the end of an era," he said.
"She epitomised that generation of British people who endured the Second World War with both bravery and stoicism.
"Her long reign, the longest in British history, saw profound change affecting virtually every aspect of politics and society. Throughout her reign, the Queen was a constant and reassuring presence in British life.
"Her historic visit to Ireland in 2011, the first by a reigning British monarch since our independence, was a critical step in the peace process and helped to move British-Irish relations to a new level.
"The warmth and spontaneity of the Queen on that occasion touched all those she met. So did her generosity of spirit and her commitment to reconciliation on these islands.
"On behalf of the Government, I was privileged to welcome her to the English Market in Cork and to witness how much that visit meant to her personally and to the people of Cork.
"Through our Embassy in London, I have conveyed deepest sympathies to His Majesty, King Charles, the Royal family more generally and to all the British people on their sad loss.
"While they grieve that loss, they also can give thanks for a long life of public service lived to the full with immense dignity."
Lady Chief Justice of Northern Ireland, Dame Siobhan Keegan, added: “I am very saddened this evening by the news of the death of Her Majesty the Queen.
"Her Majesty dedicated over seven decades of her life to selfless public service in the United Kingdom and Commonwealth.
"I would like to express my condolences and that of the judiciary of Northern Ireland to the Royal Family.”
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