15 March 1961
On 5 October 1960 a referendum of White voters was held to decide whether South Africa should become a republic. The result showed that 52% were in favour of a republic. In accordance with his promise that the republic would remain within the British Commonwealth, Dr H.F. Verwoerd, South African prime minister, went to London in March 1961 to give the Conference of Commonwealth Prime Ministers formal notice that South Africa was going to change from a monarchy to a republic, and at the same time to request permission to remain within the British Commonwealth. This request was strongly opposed by African States, joined by India and Canada, not because of the proposed constitutional change, but because of South Africa's policy of apartheid. When it was clear that his request would not be granted, Verwoerd withdrew South Africa's application for membership of the British Commonwealth "in the interests of South Africa's honour and dignity". Read more about forced removals and the Supreme Court's ruling on specific areas.
References

Muller, C.F.J. (ed)(1981). Five Hundred years: a history of South Africa; 3rd rev. ed., Pretoria: Academica.|Continuity Africa, South Africa Becomes a Republic and Withdraws from the Commonwealth, from ContinuityAfrica, [online], Available at continuityafrica.com [Accessed: 17 February 2014]