28 June 1977
The United Party, main political opposition party in South Africa, was formally disbanded by the majority faction after members had left the party to join other new political parties and the ruling National Party (NP). Subsequently, the main group decided to form the New Republic Party the next day, with Sir de Villiers Graaff as interim leader.
References

Wallis, F. (2000). Nuusdagboek: feite en fratse oor 1000 jaar, Kaapstad: Human & Rousseau.|Muller, C.F.J. (ed)(1981). Five Hundred years: a history of South Africa; 3rd rev. ed., Pretoria: Academica, p. 520.|UP., (1977), 'United Party(UP) Origins and disbandment', from Encyclopaedia Britannica, 28 June, [online] Available at www.britannica.com [Accessed: 18 June 2014]