Britain's King Charles has left Balmoral Castle where he and other members of the Royal Family had gathered following the death of his mother Queen Elizabeth, according to a Reuters witness.

It is understood the rest of the royal family - including Prince William and Prince Andrew - remain at Balmoral Castle.

King Charles III and Camilla, Queen Consort, are travelling back to London from Aberdeen International Airport as the UK enters a period of mourning.

The King will hold an audience with British Prime Minister Liz Truss in the coming hours and prerecord a televised address to the nation, which will be broadcast at 6pm local time.