Japan's Princess Mako finally marries her university sweetheart Kei Komuro on Tuesday, but it won't be a lavish affair, with the couple forgoing traditional rites after years of controversy.

Emperor Naruhito's niece Mako, who turned 30 over the weekend, will lose her royal title when she and Komuro register their marriage.

This is par for the course for female members of the lineage, who cannot ascend the Chrysanthemum Throne and must leave the imperial family when they marry a commoner.

But, for the first time in Japan's post-war history, Mako has turned down a large payment offered to royal women on their departure, and the nuptials will be officiated on paper with no elaborate ceremony.

Mako and 30-year-old Komuro, who works for a US law firm, announced their engagement in 2017 -- all bashful smiles as he called her "the moon" quietly watching over him and she compared his smile to the sun.

However, the lovebirds soon found themselves the subject of tabloid gossip as a scandal erupted around reports that Komuro's family had run into financial difficulties.

Japan's royals are held to exacting standards, and the Imperial Household Agency recently said Mako had developed complex post-traumatic stress disorder because of the media attention.

The couple postponed their marriage and Komuro moved to New York for law school in 2018, a move seen as a bid to defuse negative attention.

He only returned to Japan last month, sporting a headline-grabbing ponytail.

The recent graduate emerged from his mandatory two-week quarantine for overseas arrivals, ready for the long-delayed announcement of his marriage to Mako, daughter of Crown Prince Akishino.

No crimson kimono

The low-key proceedings stand in contrast to those of another royal to marry out of the family: Ayako, the youngest daughter of former emperor Akihito's late cousin.

At her wedding in 2018, she wore a crimson-coloured kimono robe for female aristocrats, with her hair swept back in a ponytail in a traditional style.

But for Mako and Kei, a "wedding ceremony, reception banquet and other rituals won't be held, and a lump-sum payment won't be provided", the Imperial Household Agency said this month, referring to a conventional gift reportedly worth up to 153 million yen ($1.35 million).

The couple is said to be planning a move to the United States after the marriage, drawing inevitable comparisons with another royal couple who have faced a media onslaught: Britain's Prince Harry and Meghan Markle.

It is not clear whether Mako will work once there, but she is well qualified. The princess studied art and cultural heritage at Tokyo's International Christian University, where she met Komuro, and spent a year at Edinburgh University.

She also holds a Master's degree in Museum Studies from Britain's University of Leicester.

The throne can pass only to male members of the family, and the children of female royals who marry commoners are not included.

There has been some debate over changing the rules, and a government panel in July compiled notes on the issue including a proposal that royal women stay in the family, even after marriage.

However, any change to the system is likely to be a long time coming, with hardliners and traditionalists vehemently opposed to any steps towards letting women rule.