Japan's Princess Mako, the niece of the Emperor, has given up her royal title to marry her college sweetheart Kei Komuro on Tuesday.

Following the step, she revealed she was determined to build a happy life with her "irreplaceable" husband after a tumultuous engagement.

In an unusually frank joint news conference with her new husband, Mako said "incorrect" news reports about Komuro had caused her great sadness, stress and fear.

She was diagnosed with post-traumatic stress disorder (PTSD) earlier this year after a four-year engagement plagued by money scandals and intense media scrutiny.

"I'm aware that there are various views on our marriage. I feel very sorry for those (for) whom we have caused trouble ...," said Mako, who will from now be known as Mako Komuro, having had to give up her royal title after marrying a commoner, in line with Japanese law.

"For us, marriage is a necessary choice to live while cherishing our hearts"

The two, 30, were married in the morning after an official from the Imperial Household Agency (IHA), which runs the family's lives, submitted paperwork to a local office registering their marriage.