TOKYO (AFP) - Japan's Crown Princess Masako on Sunday (Dec 9) pledged to do her best despite feeling "insecure" about becoming empress in mere months, in a candid statement released on her 55th birthday.

Emperor Akihito will step down on April 30, 2019 - the first abdication in the Japanese imperial family in more than two centuries.

His eldest son and Masako's husband Naruhito will ascend to the throne the next day.

"Even though I feel insecure about how helpful I will be when I think about the days ahead... I want to devote myself to the happiness of the people, so I will make an effort to that end," said the crown princess.

The Harvard- and Oxford-educated Masako left a promising career as a diplomat to marry into the imperial family in 1993.

She gave birth to Princess Aiko in 2001 but continued to face pressure to produce a son, as only male heirs can take the throne under Japanese law.

The pressure eased in 2006, however, when her sister-in-law gave birth to Prince Hisahito, now 12.

She has made few public appearance since 2003 and has struggled with adjustment disorder.

In her birthday statement, Masako revealed she was recuperating from a stress-induced illness ahead of the imperial succession.

"I am delighted at the fact that I can perform more duties than before, as I have tried to improve my physical condition," she said.

But her doctors warned in a separate statement that it is important for the crown princess to continue treatment, as she remains vulnerable to fatigue, especially after a large ceremony or successive duties.