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."