Prime Minister Liz Truss will accompany King Charles on a tour of Britain's four nations to lead days of national mourning for his mother, Queen Elizabeth, her spokesman said on Saturday.

Truss, who was appointed prime minister by Elizabeth on Tuesday in her last public act, was one of several senior lawmakers who swore a new oath of allegiance to the new king in a special session in parliament earlier on Saturday following the queen's death.

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 colorful ceremony laden with pageantry and tradition.

She later met 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.