Japan's Emperor Naruhito and Empress Masako received a guard of honour and royal welcome from King Charles III in London on Tuesday at the start of a three-day state visit to the UK.

The couple, who arrived on Saturday and held informal engagements over the weekend, were formally greeted by the British monarch, Queen Camilla and others ahead of a lavish evening banquet.

It is the first state visit to the UK by a Japanese head of state since 1998 and, unusually for such a visit, comes during a general election campaign.

Some typical political elements, such as a Downing Street visit, have been scrapped in line with neutrality rules but all the traditional pomp and pageantry remained.

The emperor and empress were met on Horse Guards Parade in central London by a host of dignitaries -- including Prime Minister Rishi Sunak -- while gun salutes were fired and the two countries' national anthems were played.

After Naruhito, 64, and Charles, 75, inspected the guard of honour, the royals travelled the short distance to Buckingham Palace in a carriage procession for lunch.

The king, hosting his first state visit since it was revealed in February that he has cancer, will later take the emperor and empress to a special exhibition of items from the Royal Collection relating to Japan.

They will also lay a wreath at Westminster Abbey at the Grave of the Unknown Warrior.

- Banquet -

Charles is hosting a state banquet at Buckingham Palace -- the showpiece of a state visit to the UK -- on Tuesday evening.

Sunak and Labour Party leader Keir Starmer, who is widely expected to win the July 4 election, are attending.

All eyes, though, are on a possible appearance by heir to the throne Prince William's wife Catherine, who has not been seen at an official engagement since December due to her own cancer treatment.

Charles has made a limited return to public duties in recent months after doctors said they were "very encouraged" by his progress.

Princess Anne, the king's sister, had been due to attend but will now not be there as she recovers in hospital with concussion and minor injuries after apparently being injured by a horse.

The trip is the emperor's second official state visit since he ascended to the Chrysanthemum Throne in 2019, following a trip to Indonesia last year.

It was originally due to take place in 2020 and would have been the emperor's first overseas visit but it was delayed by the pandemic.

For Charles, it will be the third state visit he has hosted since he became king following the death of his mother Queen Elizabeth II in September 2022.

It was announced Tuesday that the UK monarch had approved an honours award for Nissan CEO Makoto Uchida -- making him an Honorary Commander of the Most Excellent Order of the British Empire (CBE) -- for services to UK-Japan business relations.

- 'Fond memories' -

Naruhito will visit The Francis Crick Institute, the UK's flagship biomedical research centre, on Wednesday before the Lord Mayor and City of London Corporation host a banquet for him in the evening.

The emperor and empress will formally bid farewell to the king and queen at Buckingham Palace on Thursday morning before attending the "Japan: Myths to Manga" exhibition at London's V&A museum.

The emperor will later privately visit Windsor Castle to lay a wreath on the tomb of the late queen, whose funeral he attended nearly two years ago.

Elizabeth, whose 70-year reign began in 1952, hosted two Japanese state visits during her reign: Emperor Hirohito in 1971 and his eldest son Emperor Akihito -- Naruhito's father -- in 1998.

Speaking 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 (her husband) Prince Philip showing me around by driving a carriage himself," he told a rare press conference.

The Japanese couple head to Oxford, west of London on Friday, and will fly home from near there.