Liz Truss has led tributes to the Queen during a special sitting of parliament saying the late monarch was "devoted to the union".
The Prime Minister insisted Britain must show it does not fear what lies ahead after "our lives changed forever" following the death of its "icon" Queen.
She said the King "bears an awesome responsibility that he now carries for all of us", adding: "Even as he mourns, his sense of duty and service is clear."
Her words were heard in a crowded but emotionally charged and silent chamber, the green benches populated by MPs dressed in black, as Parliament came together to mark the death of the nation's longest-reigning monarch.
She said: "The United Kingdom is the great country it is today because of her. The Commonwealth is the family of nations it is today, because of her. She was devoted to the union of England, Scotland, Wales and Northern Ireland.
"She served 15 countries as head of state, and she loved them all. Her words of wisdom gave us strength in the most testing times. During the darkest moments of the pandemic, she gave us hope that we would meet again. She knew this generation of Britons would be as strong as any."
Labour leader Keir Starmer led tributes on behalf of his party saying the Queen did not simply "reign over us", she lived "alongside us".
He said: "All our thoughts are with her beloved family, our Royal Family, at this moment of profound grief.
"This is a deep and private loss for them, yet it's one we all share because Queen Elizabeth created a special, personal relationship with us all.
"That relationship was built on the attributes that defined her reign: her total commitment to service and duty, a deep devotion to the country, the Commonwealth, and the people she loved. In return for that, we loved her."
Starmer added: "And it is because of that great shared love that we grieve today. For the 70 glorious years of her reign, our Queen was at the heart of this nation's life.
"She did not simply reign over us, she lived alongside us, she shared in our hopes and our fears, our joy, and our pain. Our good times and our bad."
Liz Truss' speech in full
"In the hours since last night shocking news we have witnessed the most heartfelt outpouring of grief for the loss of her late Majesty the Queen. Crowds have gathered, flags have been lowered to half mast, tributes have been sent from every continent around the world.
"On the death of her father, King George VI, Winston Churchill said the news had stilled the clatter and traffic of 20th century life in many lands. Now 70 years later, in the tumult of the 21st century, life has paused again.
"Her late Majesty Queen Elizabeth II was one of the greatest leaders the world has ever known. She was the rock on which modern Britain was built. She came to the throne at just 25 in a country that was emerging from the shadow of war. She bequeathed a modern, dynamic nation that has grown and flourished under her reign.
"The United Kingdom is the great country it is today because of her. The Commonwealth is the family of nations it is today, because of her. She was devoted to the union of England, Scotland, Wales and Northern Ireland.
"She served 15 countries as head of state, and she loved them all. Her words of wisdom gave us strength in the most testing times. During the darkest moments of the pandemic, she gave us hope that we would meet again. She knew this generation of Britons would be as strong as any.
"As we meet today, we remember the pledge she made on her 21st birthday to dedicate her life to service. The whole house will agree never has a promise been so completely fulfilled. Her devotion to duty remains an example to us all. She carried out 1000s of engagements. She took her red box every day. She gave her assent to countless pieces of legislation, and was at the heart of our national life for seven decades.
"As the supreme governor of the Church of England she drew on her deep faith. Her visits to post apartheid South Africa and to the Republic of Ireland, showed her unique ability to transcend difference and heal division. In total, she visited well over 100 countries.
"She met more people than any other Monarch in our history. She gave counsel to prime ministers and ministers across government. I have personally greatly valued her wise advice.
"Only last October I witnessed firsthand how she charmed the world's leading investors at Windsor Castle. She was always so proud of Britain and always embodied the spirit of our great country who remain determined to carry out her duties, even at the age of 96.
"It was just three days ago of Balmoral that she invited me to form a government and become her 15th Prime Minister. Again, she generously shared with me her deep experience of government, even in those last days.
"Everyone who met her will remember the moment they will speak of it for the rest of their lives. Even those who never met her, her late Majesty's image is an icon for what Britain stands for as a nation, on our coins on our stamps, and in portraits around the world.
"Her legacy will endure through the countless people she met, the global history she witnessed and the lives that she touched. She was loved and admired by people across the United Kingdom and across the world.
"One of the reasons for that affection was her sheer humanity. She reinvented monarchy for the modern age. She was a champion of freedom and democracy around the world.
"She was dignified but not distant. She was willing to have fun whether on a mission with 007, or having tea with Paddington Bear.
"During her first televised Christmas message in 1957, she said: 'Today we need a special kind of courage, so we can show the world that we are not afraid of the future'.
"We need that courage now. In an instant yesterday, our lives changed forever. Today, we show the world that we do not fear what lies ahead.
"We send our deepest sympathy to all members of the royal family. We pay tribute to our late Queen, and we offer loyal service to our new King. His Majesty King Charles III has an awesome responsibility that he now carries for all of us.
"I was grateful to speak to His Majesty last night and offer my condolences even as he mourns his sense of duty and service is clear. He has already made a profound contribution to his work on conservation, education and his tireless diplomacy. We owe him our loyalty and devotion. The British people, the Commonwealth and all of us in this house will support him as he takes our country forward to a new era of hope and progress - our new Carolien age.
"The crown endures, our nation endures and in that spirit I say, God to Save the King."
To sign up to the Daily Record Politics newsletter, click here.
