The Constitution of 1961 (formally the Republic of South Africa Constitution Act, 1961) was the fundamental law of South Africa for two decades. Under the terms of the constitution South Africa left the Commonwealth and became a republic.

Legally, the Union of South Africa, which had existed since 1910, came to an end and was re-established as the "Republic of South Africa".

The 1961 Commonwealth Prime Ministers' Conference was the 11th Meeting of the Heads of Government of the Commonwealth of Nations. It was held in the United Kingdom in March 1961, and was hosted by that country's Prime Minister, Harold Macmillan.

While Commonwealth conferences were normally held biennially, this conference was held after an interval of only a year as the May 1960 conference due to disagreement over South Africa and whether the country should be removed from the Commonwealth due to its policy of racial segregation with Malaya's Prime Minister demanding South Africa's expulsion.

South African Prime Minister H.F. Verwoerd, attended the conference to give formal notice that his country was to become a republic in May 1961 after having approved the constitutional change in an October 1960 referendum. South Africa's application was opposed by the leaders of African states under black majority rule, as well as Indian Prime Minister Jawaharlal Nehru, Malaya's Tunku Abdul Rahman, and the other non-white Commonwealth countries as well as Canadian prime minister John Diefenbaker due to South Africa's policy of apartheid. Canada was the only member of the old white Commonwealth to oppose South Africa's application. The "Keep South Africa In" group included Britain's Harold Macmillan, Rhodesia and Nyasaland's Roy Welensky, Australia's Robert Menzies and Keith Holyoake of New Zealand. Canadian prime minister John Diefenbaker proposed that South Africa only be re-admitted if it joined other states in condemning apartheid in principle. Once it became clear that South Africa's membership would be rejected, Verwoerd withdrew his country's application and left the conference.Concerns were also expressed about Britain's prospective membership in the Common Market and the possible impact on trade relations between the United Kingdom and the Commonwealth. The Commonwealth also expressed its support for worldwide disarmament "subject to effective inspection and control".Cyprus' application to join the Commonwealth, following its independence the previous year, was approved over the opposition of the United Kingdom which objected as Cyprus had not applied for membership prior to independence as had been customary. Cyprus' President, Archbishop Makarios III, joined the conference once the decision on his country's membership was made. The membership application of Sierra Leone was also accepted and became effective upon its independence on 27 April.

This was the first Commonwealth conference in which one of the heads of government was a woman, Sirimavo Ratwatte Dias Bandaranaike, who was also the first female prime minister in the world.