Britain's new King Charles on Thursday said the death of his mother Queen Elizabeth II is a "moment of the greatest sadness" for him.

"We mourn profoundly the passing of a cherished Sovereign and a much-loved Mother," Charles said in a statement.

"I know her loss will be deeply felt throughout the country, the Realms and the Commonwealth, and by countless people around the world," the statement added.

"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," said the new king.

Britain's Prime Minister Liz Truss also paid tribute to the late Queen Elizabeth II on Thursday and offered loyalty to Charles III, the country's new king.

Speaking to the press, Truss said: "Today the crown passes as it has done for more than 1000 years to our new monarch, our new head of state, His Majesty, King Charles III."

"We offer him our loyalty and devotion just his mother devoted so much to so many for so long. And with the passing of the second Elizabethan age, we usher in a new era in the magnificent history of our great country, exactly as her Majesty would have wished, by saying the words God save the King," she added.

In a Twitter post, Truss said: "Queen Elizabeth II was the rock on which modern Britain was built. Our country has grown and flourished under her reign."

"In the difficult days ahead, we will come together with our friends across the United Kingdom, the Commonwealth and the world to celebrate her extraordinary lifetime of service," she added.

The U.K. saw 15 prime ministers under Elizabeth's long reign, from the stately but controversial Winston Churchill to the newest Conservative leader Liz Truss.

Earlier, Buckingham Palace announced that the queen "died peacefully" at Balmoral Castle in Scotland.

"The King and The Queen Consort will remain at Balmoral this evening and will return to London tomorrow," a palace statement said.

Elizabeth II, 96, was the longest-serving monarch in British history.