Britain's newly crowned King Charles pledged to give lifelong service to the people of the UK and his other realms in a solemn address to the nation a day after the death of his mother Queen Elizabeth II.

"I speak to you today with feelings of profound sorrow," said Charles, who became king on his mother's death - head of state of the UK and 14 other realms including Australia, Canada, Jamaica, New Zealand, and Papua New Guinea.

"Throughout her life, Her Majesty The Queen – my beloved Mother – was an inspiration and example to me and to all my family, and we owe her the most heartfelt debt any family can owe to their mother; for her love, affection, guidance, understanding and example."

"As the queen herself did with such unswerving devotion, I too now solemnly pledge myself, throughout the remaining time God grants me, to uphold the constitutional principles at the heart of our nation," the King said.

"And wherever you may live in the UK, or in the realms and territories across the world, and whatever may be your background or beliefs, I shall endeavour to serve you with loyalty, respect and love, as I have throughout my life."

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King Charles bestowed on his eldest son William and daughter-in-law Kate the titles of Prince and Princess of Wales, which he and his late wife Diana previously held.

Diana was a hugely popular figure who was under constant media scrutiny from her wedding onwards and the subject of a massive outpouring of grief when she died aged 36 in a car crash in 1997, five years after she and Charles separated.

A royal source said Kate appreciated the history associated with the title but would seek to create her own path as Princess of Wales.

Delivering his first speech to the nation since the death of his mother Queen Elizabeth on Thursday, Charles said he was proud to make his heir William the Prince of Wales, a title Charles had held since 1958.

"With Catherine (Kate) beside him, our new Prince and Princess of Wales will, I know, continue to inspire and lead our national conversations, helping to bring the marginal to the centre ground where vital help can be given," he said.

William and Kate, both 40, have taken on central roles within the royal family in recent years, appearing regularly in public and increasingly taking their three young children to events, such as the Queen's Platinum Jubilee earlier this year.

On a final note, he expressed his love for Prince Harry and Meghan, his son and daughter-in-law, during his first address as monarch on Friday, a significant gesture toward the couple whose relationships with the rest of the family have been strained.

"I want also to express my love for Harry and Meghan as they continue to build their lives overseas," Charles said in a solemn televised speech.

Charles will officially be proclaimed King on Saturday at a meeting of the Accession Council held at St. James' Palace followed by proclamations across the nation.