(Reuters) - The ACT college-entrance exam was canceled Saturday for all test-takers in South Korea and Hong Kong in the latest example of how an East Asia cheating epidemic is roiling American higher education.

College students attend a class at a private cram school in Busan, about 420 km (261 miles) southeast of Seoul October 28, 2013. REUTERS/Kim Hong-Ji

The incident marked the first known cancellation of the high-stakes exam for an entire country, according to ACT spokesman Ed Colby.

ACT Inc, the Iowa-based nonprofit that operates the test, said it halted the test after discovering that it had leaked in advance.

“ACT has just received credible evidence that test materials intended for administration in these regions have been compromised,” ACT said in a statement.

The cancellation came only hours before students were to take the exam, which is used by most U.S. colleges to assess applicants.

Colby declined to discuss how the test had leaked or where. He said ACT discovered evidence of the breach on Friday.

Colby said the cancellation affected about 5,500 students who were scheduled to take the test at 56 different test centers. They will receive refunds of registration fees. He said it was “not feasible” to reschedule the exam; the ACT will not be administered again until September.

“It impacts innocent students who had no involvement in any kind of wrong activities,” he said.

South Korea has become notorious for standardized-test cheating rings. In May 2013, the College Board, which administers the rival SAT college-entrance exam, was forced to cancel its test for the entire country because exam materials had leaked. Tutors and owners of private South Korean test-cram companies currently are on trial in Seoul in that incident.

Test security issues are not limited to South Korea. As Reuters detailed in March, a vibrant industry in China has been exploiting the College Board’s practice of recycling SAT test forms by acquiring past exams and feeding their clients test questions in advance.

Since October 2014, the College Board has delayed issuing test scores in Asia six times and canceled an exam sitting in two locations there – steps that the New York-based not-for-profit takes when it has evidence that test materials have been compromised.

Overseas demand for taking the ACT and the SAT is rising as more and more foreign students seek entry into American universities. The ACT surpassed the SAT in the United States as the most widely taken college entrance test in 2012. The ACT trails the SAT outside America, but has been growing rapidly abroad in recent years, according to ACT officials.

(Story refiles to fix name to say ACT, not ATC in third paragraph.)