Japanese Princess Mako, the eldest granddaughter of Emperor Akihito, and her longtime boyfriend from university days Kei Komuro will be formally engaged in March in a traditional court ceremony, an Imperial Household Agency source said Tuesday.

The traditional rite of betrothal called "Nosai no Gi" will likely be held on March 4, which falls on a Sunday, when Komuro, who works as a paralegal at a law firm, will be off duty. Their wedding ceremony is expected in fall next year, the source said.

During the ceremony, a messenger from Komuro will visit the princess' imperial residence with gifts including sake, sea bream and textiles to formally convey the proposal. The date of their wedding will be fixed after the engagement ceremony.

Princess Mako, the 26-year-old daughter of Prince Akishino and Princess Kiko, and Komuro, 26, announced in September their planned engagement.

GALLERY: Portrait of lives of Princess Mako, her fiance Kei Komuro

GALLERY: The life of Princess Mako of Japan

In Japan, a female member of the imperial family loses her status after marrying a commoner. The government will hold a conference to decide on a lump sum allowance for Princess Mako upon her departure, which will reduce the number of imperial family members, including the emperor, to 18.

Emperor Akihito, who will likely abdicate in March 2019, is expected to attend the wedding ceremony with Empress Michiko, becoming the first emperor to take part in a grandchild's marriage ceremony in modern Japan.

Princess Mako's 22-year-old sister Kako, who is now studying at the University of Leeds in Britain, is also expected to attend the wedding ceremony as she will return to Japan next summer.

It will be the first imperial marriage since 2014 when Princess Noriko, a daughter of Emperor Akihito's late cousin Prince Takamado, tied the knot with Kunimaro Senge, the eldest son of the chief priest of Izumo Taisha, a Shinto shrine in Shimane Prefecture.

Princess Mako will be the first "naishinno" princess, a daughter or granddaughter of the emperor, to marry since the emperor's only daughter Sayako Kuroda in 2005.