In a majority ruling, Chief Justice Mogoeng Mogoeng denied AfriForum's application for leave to appeal a Supreme Court of Appeal (SCA) ruling from March. The SCA overruled an earlier High Court order which said the University's policy decision was unlawful.
In a statement, AfriForum's deputy CEO Alana Bailey said minorities, such as Afrikaans speakers, were misled in 1994 to believe that their language rights would be protected.
Afrikaans is South Africa's third most common language, with an estimated seven million speakers.
Bailey expressed fear that the ruling will heighten racial tension on South African campuses.
"The South African past (consider for example the events in Soweto in 1976), but also many other countries such as Bangladesh and Belgium, prove that denying students the right to study in their mother language might lead to increased tensions and even violence," Bailey said.
"With English monolingualism, only a tiny group of English-speaking students will be privileged, while the rest will have very little hope left that any indigenous language will develop further in future."
The FF Plus, which advocates for Afrikaners to have the right to self-determination or self-management, said the Constitutional Court ruling is a "tremendous setback for mother-tongue instruction".
"In a country with eleven official languages, the mediums of instruction must rather be expanded to include more languages instead of languages being taken away and institutions becoming anglicised," FF Plus chairperson Anton Alberts said in a statement.
Others welcome judgment
While AfriForum and FF Plus expressed disappointment in the ruling, the South African National Civic Organisation (Sanco) and the Higher Education Transformation Network (HETN) welcomed the judgement.
In a statement, SANCO spokesperson Jabu Mahlangu said AfriForum opposed to the new language policy because, except for the "preservation and domination of the Afrikaans language, it has no interest whatsoever in peaceful solutions to any challenge facing South Africa".
"[AfriForum] is a reactionary formation that is part of the right-wing movement that thrives on heightened racial tensions to appeal to those who wish to plunge the country towards a slippery slope as well as a vicious cycle of conflict, racial hatred and violence,” Mahlangu explained.
On their part, HETN said the ruling should be considered the "official flattening of AfriForum’s racist campaign to retain Afrikaans as the sole medium of instruction in formerly Afrikaans–only public higher educational institutions".
"Whilst it may not be possible for the government to provide for indigenous language education for all, English should remain the international standard medium of instruction to ensure that all students from all South African communities are able to access higher education equally," HETN Executive Director Mothepane Seolonyane added.
The language of instruction at formerly Afrikaans universities such as the University of Free State (UFS), Stellenbosch University (SU) and the University Of Pretoria (UP) have come under fire recent years.
UFS and UP have opted for English-only instruction as a means to assist in its transformation, while SU gave Afrikaans and English equal status as languages of instruction.
SU's policy, however, adopts a preference for English in certain circumstances in order to advance the university’s goals of equal access, multilingualism and integration.