Sarah Ferguson paid touching tribute to the Queen - saying she was the "most incredible mother-in-law and friend".
In a tribute on Instagram she wrote: "I am heartbroken by the passing of Her Majesty the Queen. She leaves behind an extraordinary legacy: the most fantastic example of duty and service and steadfastness, and a constant steadying presence as our head of state for more than 70 years.
"She has given her whole life selflessly to the people of the UK and the Commonwealth.
"To me, she was the most incredible mother-in-law and friend. I will always be grateful to her for the generosity she showed me in remaining close to me even after my divorce. I will miss her more than words can express."
The new King Charles has paid tribute to his mum, saying: "The death of my beloved Mother, Her Majesty The Queen, is a moment of the greatest sadness for me and all members of my family.
"We mourn profoundly the passing of a cherished Sovereign and a much-loved Mother. 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."
New Prime Minister Liz Truss also paid tribute to the Queen, saying she was the country's "rock".
Truss said: "Today the Crown passes, as it has done for more than a thousand years, to our new monarch, our new head of state, his majesty King Charles III."
The Prime Minister ended her speech saying "Long Live the King".
In a solemn message to the nation, Truss added: "Queen Elizabeth II was the rock on which our country was built.
Operation Unicorn is now ready as the country's longest-reigning monarch dies in her favourite residence, which will likely see the neighbouring palace of Holyrood house and St Giles' Cathedral as the main places for the public to pay their respects.
According to the plans, during a period of mourning, her body will be moved from Balmoral to Holyrood house in Edinburgh before travelling to London via train.
Buckingham Palace made the announcement at 6.30pm, stating: "The Queen died peacefully at Balmoral this afternoon. The King and The Queen Consort will remain at Balmoral this evening and will return to London tomorrow."
The UK is now in a period of official mourning with her funeral due to take place within a fortnight.
The Queen and Prince Philip's four children – Charles, the Princess Royal, the Duke of York and the Earl of Wessex – were at her bedside, joined by the Duchess of Cornwall and the Countess of Wessex.
