Charles III became the king of Great Britain

Queen Elizabeth II, the longest-serving monarch in British history and an icon instantly recognisable to billions of people around the world, died at her Scottish Highland retreat on Thursday. She was 96.

The eldest of her four children, Charles, Prince of Wales, who at 73 was the oldest heir apparent in British history, became king immediately.

Royal officials confirmed he is now known as King Charles III -- the first king of that name to sit on the throne since 1685.

His mother's death triggered an outpouring of condolences across the globe as world leaders paid homage to a woman whose reign spanned 70 years, straddling two centuries of seismic social, political and technological upheaval.

Hundreds of flowers and flickering candles were left at the gates of Buckingham Palace in central London, as billboards lit up in tribute across the capital with images of the queen.

In his first words as monarch, Charles called her death "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," he added in a statement signed "His Majesty the King".

"I know her loss will be deeply felt throughout the country, the Realms and the Commonwealth and by countless people around the world."

Buckingham Palace announced the queen's death in a short statement, triggering 10 days of national mourning and a global outpouring of tributes to her long life and record-breaking reign.