Japan's Emperor Naruhito said Britain's royals treated him "like family", as he recounted previous trips at a rare press conference on Wednesday ahead of a UK state visit.
But the monarch, who will travel to Britain with Empress Masako, declined to comment when questioned on the thorny issue of male-only succession rules.
Naruhito recalled that during his two years as a student at Oxford University in the 1980s, 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 said.
King Charles, then crown prince, also taught Naruhito fly fishing but "neither of us had success", he smiled.
"I was welcomed with warmth as if I were their family member."
The imperial couple arrive in Britain on Saturday for private events ahead of the June 25-27 state visit, such as meeting Japanese people living there.
The trip follows their first state visit last year to Indonesia. Naruhito and Masako also attended Queen Elizabeth II's funeral in 2022.
Official events begin with a Guard of Honour ceremony, a carriage procession and a state banquet at Buckingham Palace.
Naruhito said he was "very grateful" that King Charles will welcome the couple, despite his cancer diagnosis.
He added that he had heard Catherine, Princess of Wales, who is also being treated for cancer, was gradually returning to her duties.
"I hope their treatment goes smoothly and they can make a speedy recovery."
The schedule includes visits to museums and the Francis Crick Institute, a biomedical research centre in London.
But the pair will not go to Downing Street, because parliament has been dissolved ahead of a July 4 general election.
The couple will visit Oxford on their final day, where former high-flying diplomat Masako also studied.
Masako is still in the process of recovery from a stress-induced illness she developed after joining the household, which some have put down to the pressure of producing a male heir.
A Japanese foreign ministry official said on Tuesday the schedule was not too "dense" for her.
"She's very happy to visit the United Kingdom," the official added.
Japan is facing an imperial succession crisis, as Naruhito's 17-year-old nephew Prince Hisahito is the only current heir under male-only traditions.
His daughter Princess Aiko, 22, is barred from the throne under the Imperial Household Law, in place since 1947.
Women in the family must give up their royal status when they marry a commoner.
"The number of imperial family members who can take on official activities is decreasing compared to before, due to the fewer number of male royals, ageing members and female royals leaving the family," Naruhito said on Wednesday.
"This is a problem that is related to the future of the imperial family."
Last month, Japanese lawmakers kicked off discussions about possible relaxations to the succession rules.
But the emperor said he wanted to refrain from commenting on the system.
