Japan's Crown Princess Masako has said she feels "insecure" about assuming the role of empress next year amid an ongoing battle with stress-related illness, in comments that provide a rare window into the emotional pressures faced by the country's royal family, CNN reports.

In a candid statement to mark her 55th birthday on Sunday, December 9, Masako vowed to do her best to fulfill her duties when her husband, Crown Prince Naruhito, succeeds his 84-year-old father, Akihito, to the throne in April 2019.

"Thinking about the days to come, I feel insecure how helpful I can be," she said. "I would like to devote myself to support his highness the Crown Prince and to make an effort for the happiness of the people," Masako said.

"I would like to continue efforts for improving my health and devoting myself to public duties as much as I can participate."

Masako married Naruhito in 1993, abandoning a high-profile career as a diplomat for life in the conservative royal household.

But the demands of imperial life have proved difficult for Masako, who has long battled with an illness doctors have described as an "adjustment disorder."

Masako, who lived abroad as a child and was educated at Harvard University, had previously faced pressure to provide a royal heir to the throne. Her daughter, princess Aiko, is forbidden from becoming empress by Japan's male-only succession law.