The emperor's trip seeks to deepen the military and scientific ties between the two countries.
Britain's King Charles and Queen Camilla welcomed Japan's Emperor Naruhito and his wife to London on Tuesday, marking the start of a three-day state visit which includes a military parade, carriage procession and state banquet.
Charles' son and heir to the throne Prince William had earlier travelled to the hotel where the 64-year-old emperor and Empress Masako were staying, before the trio arrived for a ceremonial welcome at Horse Guards Parade on Tuesday.
The two heads of state, Charles wearing a top hat, then inspected the Guard of Honour as the rows of soldiers in their traditional scarlet uniforms and black bearskin hats stood in the summer heat.
They then travel onto Buckingham Palace in a gold-edged horse-driven carriage.
The emperor's trip seeks to celebrate and deepen the military, cultural and scientific ties between the two countries, and was postponed from 2020 due to the pandemic.
It is now taking place in the run up to an election in Britain on July 4, meaning that some of the usual political elements will be missing.
There will be no meeting at Downing Street between the emperor and British Prime Minister Rishi Sunak, although Sunak and opposition leader Kier Starmer will both attend the state banquet at Buckingham Palace on Tuesday evening.
The visit comes at a difficult time for the British monarchy, after Charles, 75, was diagnosed with cancer earlier this year, and as William's wife Catherine undergoes preventative chemotherapy treatment for cancer.
In another setback, Princess Anne, Charles' younger sister suffered a head injury on Sunday after an incident believed to have involved a horse. While she is expected to make a full recovery, she has pulled out of attending the state banquet.
Emperor Naruhito came to London in 2022 to attend Queen Elizabeth's funeral, and is fond of Britain, having studied here in the early 1980s.
He has spoken of the kindness the British royals showed him at that time, including a visit to the royal family at Balmoral Castle in Scotland, where Charles took him fly fishing.
The pair will have time to discuss those memories over lunch at Buckingham Palace on Tuesday. They will then view Japanese-related items from the royal collection before a tour of Westminster Abbey in the afternoon.
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