Charities have criticised the UK after the government announced it would stop direct aid to South Africa in 2015.
UK ministers said their relationship with South Africa should now be based on trade and not development.
But international charity Action Aid said it was the "wrong decision at the wrong time".
South Africa's government warned that ending the aid programme, currently worth £19m a year, would have "far-reaching implications".
It said it had not been properly consulted or informed about the move.
Action Aid accused the government of "running away from middle-income countries" and drew parallels with the UK's decision last year to withdraw aid from India.
"It seems to imply a more general UK policy of withdrawing aid from middle income countries abruptly and without the progressive phasing that would help communities adjust to new realities," spokeswoman Melanie Ward said.
Oxfam called on ministers to ensure the decision to end direct aid would not cost lives.
"Whilst South Africa should be in a position to fund its own development, there remains widespread poverty and inequality, so UK aid is still a lifeline for poor people," said Emma Seery, head of development finance and public services at the charity.
International Development Secretary Justine Greening made the announcement to end the aid programme at a conference of African ministers and business leaders in London on Tuesday.
She described South Africa as "the region's economic powerhouse and Britain's biggest trading partner in Africa".
"I have agreed with my South African counterparts that South Africa is now in a position to fund its own development," she said.
"It is right that our relationship changes to one of mutual co-operation and trade, one that is focused on delivering benefits for the people of Britain and South Africa as well as for Africa as a whole."
But South Africa said there had been no "proper consultations" about the move.
"This is such a major decision with far-reaching implications... and it is tantamount to redefining our relationship," the Department of International Relations and Co-operation said in a statement.
"The UK government should have informed the government of South Africa through official diplomatic channels of their intentions."
In response, UK officials said they had had "months of discussions" and "many meetings" with their South African counterparts prior to the decision.
Speaking on London radio station LBC, Deputy Prime Minister Nick Clegg defended the ending of aid, saying the "principle... is the right one".
Labour said ministers had taken the "wrong decision" and the South African authorities had "contradicted" Ms Greening's version of events.
Shadow international development secretary Ivan Lewis said it was important to maintain financial help to countries like South Africa, which he described as "strategically" very important.
"This looks like a serious breach of trust with one of our most important strategic partners. Justine Greening must explain why she is saying one thing about her conduct while the South African government is saying another," Labour said in a statement.
"Behaving in what looks like a high-handed and patronising fashion towards South Africa is no way to treat one of the world's key emerging nations and is not in Britain's national interest."
UK aid to South Africa is focused on reducing the mortality rate among women giving birth and supporting businesses.
At its peak in 2003 it was worth more than £40m.