Ribbon no KishiPrincess Knight - Tezuka, Osamu

Taking place in a medieval fairy-tale setting, Ribbon no Kishi is the story of young Princess Sapphire who must pretend to be a male prince so she can inherit the throne (as women are not eligible to do so). This deception begins as soon as she is born, as her father the King announces his baby is a boy instead of a girl. The reason for this is that the next-in-line to the throne, Duke Duralumon, is an evil man who would repress the people if he were to become king, and because of this the King will go to any length to prevent him from taking over.

(Source: Wikipedia)

Ribbon no Kishi was published in English as Princess Knight by Vertical on November 1 and December 6, 2011. The English version was split into two volumes, rather than the original three. A digital version was published August 12, 2015.