Prime Minister Liz Truss will join King Charles as he leads "services of reflection" around the United Kingdom, Downing Street has announced.
She is set to travel to Scotland on Monday, followed by visits to Northern Ireland on Tuesday and Wales on Friday as the new monarch goes on a tour of the country.
"The Prime Minister believes it is important to be present for what will be a significant moment of national mourning around the United Kingdom," her official spokesman said.
Ms Truss, who was appointed Prime Minister by Queen Elizabeth on Tuesday in her final public act, was one of several senior MPs who swore a new oath of allegiance to the new king in a special session in Parliament on Saturday.
She was also present, along with six former Prime Ministers, senior bishops and several politicians, when Charles, 73, was officially proclaimed Britain's new king in a ceremony laden with pageantry and tradition.
Ms Truss later met King Charles along with other senior ministers at Buckingham Palace.
"In terms of the prime minister's involvement, she will join the king as he leads the national mourning across the United Kingdom, attending services of reflection in Scotland on Monday afternoon, in Northern Ireland on Tuesday, and Wales on Friday," the spokesman told reporters.
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The Queen is set to leave her Scottish home for the last time this morning, as her coffin makes its long journey back to London.
The cortege is due to leave Balmoral where the Queen died on Thursday with her son King Charles and daughter Princess Anne beside her.
Her coffin has been draped with the Royal Standard of Scotland with a wreath of flowers on top. It has remained at rest in the Balmoral ballroom so the late monarch's loyal Balmoral estate workers could say their last goodbyes.
The oak coffin is set to be lifted into a hearse at 10am by six of the estate's gamekeepers, who have been tasked with the symbolic gesture, ready for a six-hour journey to Edinburgh.
Well-wishers are expected to line the long route as the cortege travels through Aberdeen and Dundee before arriving in Edinburgh, where the Queen will remain overnight.
* Today the Sunday Mirror celebrates the life of Her Majesty the Queen with a commemorative special filled with all the key moments from Britain’s longest reigning monarch.
