Over 30 percent of international schools in South Korea have been using textbooks that refer to the East Sea, a body of water between South Korea and Japan, only as the Sea of Japan, a lawmaker said Tuesday.

As of August 2020, 12 of 38 international schools were using textbooks that solely use the Japanese name. The number of those textbooks with the Sea of Japan reference marked 24, according to the office of Rep. Ahn Min-seok of the opposition Democratic Party of Korea. The office referenced data from the Academy of Korean Studies.

Five schools used textbooks -- 27 books in total -- that use both the Korean and Japanese names, while only two were found to be using textbooks, totaling nine, with only “Donghae,” the Korean term for "East Sea."

Korea argues that "Sea of Japan" is a relic of Japan's imperialistic past, including the 1910-45 colonization of Korea. The original and correct name is the East Sea, Koreans contend. Tokyo registered the Japanese name with the International Hydrographic Organization in the early 1920s.

On the Dokdo islets lying between South Korea and Japan, over which the two are locked in a similar name dispute, one international school was found to be using textbooks using the Japanese name “Takeshima.” Two other schools used both Korean and Japanese terms, data showed.

“It’s the government’s duty to correct errors in school textbooks regarding descriptions of the East Sea and Dokdo. Long-term measures at the national level are needed to revise improper school textbooks even though there are some challenges such as a shortage of professional inspectors or possible diplomatic tension,” the lawmaker said in a press release.

The Ministry of Education said it would send reference materials about the East Sea to international school educators as well as foreign publishing companies to encourage them to use the appropriate terms.

“Most international schools in Korea use textbooks that were published in their countries, which makes it difficult to win a revision,” an official from the ministry said.