The map was printed on page 36 of "Developing Chinese," a beginner’s Chinese reading textbook which is part of the Chinese-Japanese department curriculum for freshmen at the private university.

Bui Van Thanh, head of the department, said on Sunday that the book was published by Beijing Language and Culture University and was used by HUBT for the 2019-2020 school year.

The university chose the book because it has good content and reputation, linguistically, and the committee tasked with reviewing the book did not find any sentences which has political or sovereignty issues, he added.

"But the map was small and we did not notice its problem," Thanh said.

Vice Principal Vu Van Hoa said that the university has been recalling the books, numbering 500-700, as soon as the fraudulent map was discovered.

Some students have not yet returned their books, and have been asked to do so at the earliest, he said.

The university had bought the book from a third party that imports foreign textbooks, he said.

"We did not compose the book. If we had, this would not have happened."

There needs to be a mechanism to control the content of imported textbooks, and this is something that the government should institute, not the university, he said.

The infamous and controversial nine-dash line is a demarcation that claims 90 percent of the 3.5-million-square-kilometer East Sea, known internationally as South China Sea. The unilateral demarcation has been strongly opposed by the international community. Apart from violating Vietnam’s sovereignty, it overlaps with claims by Brunei, Malaysia, the Philippines, and Taiwan.

The textbook is the latest incident concerning the nine-dash line map to surface in Vietnam.

Vietnam demoted its director of the Cinema Department at the culture ministry on October 28 for letting the Hollywood-produced animation movie "Abominable" carrying the line to be screened in Vietnam.

On October 27, a Volkswagen SUV displayed at the Vietnam Motor Show 2019 in Ho Chi Minh City was found having the map in its navigation system.

Also last month, the Vietnam Register ordered a Vietnamese car distributor of four made-in-China vehicles, including Zotye and Baic, to remove the infamous map from their nagivation apps; while top travel firm Saigontourist was fined VND50 million ($2,200) for using brochures with the line at a tourism fair in September.