Image copyright Reuters Image caption Princess Masako, pictured with Prince Naruhito and their pet dog, said: "I will strive to do my very best"

Japan's Crown Princess Masako says she feels "insecure" about becoming empress in April, but will do her best to serve the people of Japan.

Masako Owada will become empress when her husband, Crown Prince Naruhito, succeeds his father Emperor Akihito.

Emperor Akihito, 84, is abdicating next year because of his age and health.

The princess, who has suffered from a stress-related disorder for many years, says she is slowly recovering and will try to perform more royal duties.

Princess Masako was educated at Harvard and Oxford, and had a promising career as a diplomat before her marriage in 1993, the BBC's Asia analyst Michael Bristow reports.

However, she has struggled to cope with royal life and Japan's notoriously conservative imperial household, our correspondent adds.

The princess made the comments in a statement marking her 55th birthday.

"Giving thought to the days ahead, I sometimes feel insecure about the extent to which I will be able to be of service to people," she said. "But I will strive to do my best so that I can contribute to their happiness."

She said that she was "filled with deep emotions" and felt "nostalgic" that this was the last year before Emperor Akihito abdicated.

Princess Masako's doctors say she suffers from "adjustment disorder", a condition caused by stress, that is often linked to depression or anxiety.

In Sunday's statement, she said that her health was improving slowly, adding: "I am happy as I can now perform more official duties than before, little by little."

Her doctors emphasised, in a separate statement, that it was important that the princess be allowed to continue her treatment and not be subjected to too much pressure.

The princess "is still on the recovery track and there are ups and downs to her condition", they said.

'I will protect you for my entire life'

Image copyright AFP/Getty Images Image caption The couple married in 1993

Crown Prince Naruhito reportedly met Masako Owada at a tea party for a Spanish princess in 1986.

Ms Owada, who is fluent in several languages, had just passed exams that qualified her to work as a top-ranked diplomat.

She was reportedly concerned about marrying into the royal family, but agreed to marry Prince Naruhito in 1993.

She later told reporters that she had accepted his proposal after he said: "You might have fears and worries about joining the imperial household. But I will protect you for my entire life."

The couple came under pressure to produce a male heir despite having a daughter, Princess Aiko, in 2001.

In 2004, the prince told journalists his wife had "completely exhausted herself" trying to adapt to palace life, and accused palace officials of "moves to negate" her character and career.

The princess stayed largely out of the public eye for the next decade, although she has attended more and more functions in recent years.