Japan's Emperor Naruhito and Empress Masako arrive in Britain on Saturday ahead of a three-day state visit hosted by King Charles III.
The royal couple are due to touch down on a flight from Japan and spend Sunday and Monday at private engagements before the official programme begins on Tuesday.
The highlight will be a state banquet at Buckingham Palace, with all eyes on a possible appearance by heir to the throne Prince William's wife Catherine, rarely seen in recent months due to cancer treatment.
Naruhito, 64, will also go to Windsor Castle west of London to lay a wreath on the tomb of Queen Elizabeth II, whose state funeral he and the empress, 60, attended in September 2022.
With Britain in the midst of a general election campaign, it was not immediately clear if there would be any meeting with Prime Minister Rishi Sunak.
Visiting heads of state have typically held talks with the premier and opposition leader, or addressed lawmakers in parliament.
But there are no MPs as parliament has been dissolved before the July 4 vote.
The trip will be the emperor's second official state visit since his accession to the throne in 2019, following a visit to Indonesia last year.
For Charles, 75, it will be the third state visit he has hosted since he ascended to the throne following the death of his mother.
Like Catherine, he was diagnosed with cancer earlier this year but has made a limited return to public duties, and was seen this week with his wife Queen Camilla at the Royal Ascot horse racing meeting.
Catherine, a future queen who is widely known as Kate, is currently undergoing chemotherapy and has stepped back from public life to aid her recovery.
She made a tentative return to public life last week when she attended a military parade in London to mark Charles' official birthday.
Elizabeth, whose 70-year reign began in 1952, hosted two Japanese state visits during her time on the throne: emperor Hirohito in 1971 and his eldest son emperor Akihito -- Naruhito's father -- in 1998.
Speaking in Tokyo ahead of the trip, Naruhito said Britain's royals treated him "like family" during his time studying in England in the 1980s.
Naruhito recalled that during his two years at Oxford University, he was invited to Balmoral Castle in Scotland for a few days.
"I have very fond memories of the queen driving a car and inviting me to a barbecue... and Prince Philip showing me around by driving a carriage himself," he told reporters in a rare press conference.
The state visit's programme will also include a Guard of Honour ceremony, a carriage procession at Buckingham Palace, visits to museums and to the Francis Crick Institute, a biomedical research centre in London.
Before leaving on Friday the Japanese emperor and empress will visit Oxford, where they both studied, for private engagements.
