Ryanair is requiring South African passengers to prove their nationality before travelling by completing a test in Afrikaans, a language used by just 12 percent of the population.
Europe's largest airline by passenger numbers, which does not operate flights to and from South Africa, said it required any UK-bound passengers from the country to fill in the "simple questionnaire" due to what it described as a high prevalence of fraudulent South African passports.
"If they are unable to complete this questionnaire, they will be refused travel and issued with a full refund," a spokesman for the Irish airline said.
South Africa's Home Affairs department, which has warned of syndicates selling fake passports, said it would issue a statement on the Ryanair test.
The UK High Commission in South Africa said on Twitter that the Ryanair test was not a British government requirement to enter the United Kingdom. The Irish High Commission did not immediately respond to a request for comment.
The low cost carrier said the test would apply to any South African passport holder flying to Britain from another part of Europe on the carrier. The airline did not immediately respond to a query about why it would apply to those routes, given Britain says it is not a requirement.
The test, which is already being conducted, triggered a backlash from South Africans in Johannesburg.
"It's very discriminatory to a whole host of South Africans who don't speak Afrikaans," Siphiwe Gwala told Reuters.
"They're using this (test) in a manner that is utterly absurd," Conrad Steenkamp, the chief executive officer of the Afrikaans Language Council, said when asked if this was the correct way of determining if the passengers were indeed South African.
Afrikaans is the third most spoken of 11 official languages in South Africa, used by 12 percent of the 58 million people in the country. It was considered the official language until the end of apartheid in 1994.