MPs and Peers will gathering at Westminster today to pay tribute to the Queen in a special session of condolence.
The House of Commons and House of Lords are due to sit at noon to allow members to pay their respects as normal business is put on hold.
Prime Minister Liz Truss will lead tributes in the Commons which are due to last until 10pm. There will also be a rare Saturday sitting, where senior MPs will take an oath of allegiance to the King from 2pm, with condolences continuing again until 10pm.
The latter session will end with a "formal humble address" to the King, "expressing the deep sympathy of the House" following his mother's death at Balmoral on Thursday, the House of Commons said in a statement.
Every MP will have the option of taking an oath to the King when the House returns but are not obliged to. Parliament's tributes will follow an outpouring of grief from across the political spectrum as the world digested news of the Queen's death at the age of 96.
The Scottish Parliament has suspended all business as a mark of respect for the Queen.
Presiding Officer Alison Johnstone said: "On behalf of the Scottish Parliament, I convey our deepest and most sincere condolences to His Majesty King Charles III and the Royal Family following the death of Her Majesty Queen Elizabeth. This is a day of great sadness for the whole country and a time of deeply personal grief for the Royal Family.
"The Queen was an extraordinary woman, who led an extraordinary life of service. From the day that this Parliament was established in 1999, Her Majesty showed her unwavering support for the institution and the time and importance she gave to that relationship is hugely appreciated.
"The Parliament will have an opportunity to consider a Motion of Condolence and reflect on the life of this much loved and much respected Monarch whose enduring bond with Scotland and the Scottish people will long be remembered."
Truss hailed the country's longest-serving monarch as the "rock on which modern Britain was built", while Sir Keir Starmer, the leader of the Opposition, said she was "a symbol of the best of us".
Boris Johnson was among the six living former prime ministers of the Queen's reign to pay his respects, two days after he met her at Balmoral to resign from office.
He said the Queen spread "magic around her kingdom" for an "unrivalled" 70 years and had a "simple power to make us happy".
Commons Speaker Sir Lindsay Hoyle will determine the timetable in the House for the following days, but it is expected to be significantly reduced until after the state funeral as Parliament adjourns.
This means new laws cannot be passed until Parliament returns, though it could be recalled for pressing matters. Buckingham Palace said the Queen died "peacefully". Her eldest son has become King Charles III.
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