South African HIV campaign group Treatment Action Campaign says the health minister has excluded it from a United Nations discussion on HIV/Aids.
The UN's envoy on Aids in Africa has expressed support for the TAC.
South Africa is thought to have between 5m and 6m of people living with HIV - the highest in the world - representing over 12% of the population.
The TAC has often clashed with Health Minister Manto Tshabalala-Msimang over her approach to Aids.
Health ministry spokesmen were not immediately available for comment.
"TAC and the Aids Law Project [at the University of the Witwatersrand] have learnt that we have been excluded from accreditation by the United Nations General Assembly Special Session on Aids (UNGASS)," a TAC statement said.
TAC enjoys credibility with everyone - apart from the South African government
Stephen Lewis, UN special envoy
"We suspect that this occurred because the South African government objected to our participation."
The statement said TAC and ALP had been on a list of organisations submitted by UNAids - the UN agency dealing with HIV/Aids - for participation in the general assembly session.
National governments have a right of veto over this list.
"We suspect that TAC and the ALP were excluded by the South African government," the TAC said.
TAC spokesman Mark Heywood later told the South African Press Association the minister herself was responsible for the ban, since other senior officials he had spoken to had no knowledge of the decision.
UN special envoy on HIV/Aids in Africa Stephen Lewis said it was "absolutely outrageous" that the TAC had been excluded from the global gathering.
"The TAC is the single most credible non-governmental Aids organisation in the world," he said.
"It carries enormous credibility with NGOs and governments and enjoys credibility with everyone - apart from the South African government."
The TAC has led the campaign for South African government clinics to provide free anti-retroviral drugs, which help people with HIV to live healthy lives.
Under such pressure, the government began supplying ARV drugs at some clinics in 2004, but deep differences remain between the TAC and the government, principally over the health ministry's support for vitamin supplements as a means of treating HIV.
Mrs Tshabalala-Msimang has suggested that those with HIV should eat more beetroot and garlic.
Story from BBC NEWS:
Published: 2006/04/03 11:35:49 GMT
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