Leader of South Africa's Economic Freedom Fighters (EFF) party, Julius Malema, has said his party is going through a "testing moment" after deputy president Floyd Shivambu resigned to join former president Jacob Zuma's uMkhonto we Sizwe (MK) party.

Shivambu, also a co-founder of the EFF, told media on Thursday that the decision to join the rival MK Party should not be read as a "vote of no confidence" in the EFF which admitted it is now expecting more members loyal to Shivambu to follow him.

In his resignation letter, Shivambu said not renewing his membership with the EFF was a revolutionary act that will allow progressive forces to unite.

"I will never abandon the cause for economic emancipation and true freedom in our lifetime," he said.

A distraught-looking Malema equated the pain of losing Shivambu to what he felt following the death of his mother.

"When he (Shivambu) sent me a letter yesterday, I felt the same pain when I received the news of the passing away of my mother. Because Floyd to me is not just a comrade - is a brother and will remain a brother even when he pursues his political career differently," Malema said.

He admitted that the EFF had lost a "valuable member".