The 43-year-old princess withdrew from official duties more than three years ago and has been a semi-recluse since.

"It is a long trip and I have to attend a string of events so according to the advice of our doctors this may be difficult for her," said the prince, who is preparing for an eight-day visit to Mongolia next week. "Therefore, we decided that it is best I travel alone. Not only [the princess] herself, but I also regret it."

Princess Masako was officially diagnosed with an "adjustment disorder" caused by "her special status as Crown Princess" in July 2004, after she dropped out of public view amid rumours that she had suffered a nervous breakdown. The condition is associated with depression and stress and is being treated with drugs and psychiatric counseling.

Despite re-emerging from behind the palace gates for limited official duties over the past two years, speculation - frequently bizarre - persists about her state of mind. A magazine recently reported that she was about to flee her marriage to live abroad with her daughter Princess Aiko.

Earlier this year, an Australian journalist, Ben Hills, sparked controversy with his book Princess Masako: Prisoner of the Chrysanthemum Throne, which said the former diplomat was suffering from deep depression triggered by psychological abuse from her conservative imperial handlers. Mr Hills called the former diplomat's marriage to Prince Naruhito "the story of a romance gone wrong," titled a section on Prince Naruhito "mummy's boy" and described his book as "an Oriental Charles and Diana story."

Japan's Foreign Minister attacked the book's "coarse logic" and called it "insulting to the Japanese people and the Imperial family". A Japanese publisher pulled the plug on a translation after the controversy erupted.

The prince himself added to the rumour mill about his wife two years ago at a similar press conference when he stunned journalists by criticising the Imperial Household for "denying her individuality." But there were no such fireworks yesterday and, with nervous bureaucrats hovering nearby, the prince gave only the broadest clues about his wife's mysterious condition, which experts say would normally be expected to clear up in six months.

"An adjustment disorder lasts only six months in acute cases," he said, hinting it would be some time before Masako was able to accompany him on foreign trips. "Attending these international conferences is an important way of contributing to society. That is why we'd like to keep on trying [to attend more]."

Many believe the princess has struggled to adjust to life inside the cloistered imperial household since she married Naruhito in 1993. Some right-wing commentators have suggested that the princess has already recovered and is "sulking".