(Adds that incident is still under investigation and is confined to international test centers; background on cheating setbacks)
Sept 7 (Reuters) - ACT Inc, the maker of the United States' most popular college entrance exam, said on Thursday it has canceled the ACT exam scheduled for Saturday at some of its international test centers due to a breach of the test materials.
ACT, which has been the target of widespread cheating at overseas centers, has notified affected students, who will receive instructions on how to reschedule their test, a spokesman said.
ACT said it could not give specifics as to how the test materials were leaked because the incident was still under investigation.
"The incident we are talking about here is confined to specific international test centers," company spokesman Ed Colby added in an email.
Would-be test-takers from countries including China, Thailand and Australia expressed outrage on social media over the cancellation because the test, the first of the new school year, was to be used by high school seniors applying to colleges this fall.
The ACT and its rival, the SAT, are used by thousands of U.S. colleges to help choose among millions of student applicants. Both have been swamped by cheating abroad.
ACT, an Iowa City, Iowa-based nonprofit, has suffered major security setbacks in the past year. After the October sitting of the exam, ACT canceled scores for an unspecified number of students in Asia and Oceania on the writing section of the test because of a leak. Last June, the exam was canceled for all test takers in South Korea and Hong Kong due to another breach. (Reporting by John McCrank in New York; Editing by Jonathan Oatis)