Four in 10 foreigners who initially came to South Korea on a visa mostly granted to Korean language program students ended up becoming illegal immigrants, data showed Tuesday.

According to data sourced from the Ministry of Justice and released by Rep. Cho Jung-hun of the Transition Korea party, out of a total of 64,904 foreign nationals who entered Korea on the D-4 General Trainee visa, approximately 40 percent (25,898 individuals) eventually became illegal immigrants, as of June this year.

The D-4 visa is known as a training visa, issued to foreign nationals who wish to research, study or train at an educational organization, academic research center or private enterprise in Korea. It is mostly granted to those who pursue Korean language courses.

The largest proportion of those who transitioned to illegal status came from Vietnam, numbering 22,760, followed by 1,054 individuals from Uzbekistan and 820 from Mongolia.

The Justice Ministry classifies foreigners as illegal status when they are no longer reachable by authorites and whose whereabouts are unknown.

The data also showed that 27.3 percent (2,790 individuals) of foreign entrants with the D-2-1 visa, which permits foreign nationals to pursue an associate degree, became illegal immigrants.

Rep. Cho emphasized the need for a comprehensive review of the foreign student policy and implementation of rigorous visa screening processes.