The deputy leader of South African opposition party the Economic Freedom Fighters (EFF) said on Thursday he was resigning to join the party led by former president Jacob Zuma, uMkhonto we Sizwe (MK).

Floyd Shivambu's departure is a blow to the Marxist EFF, headed by Julius Malema. The EFF saw its share of the vote fall to below 10% in May's national election, as the newly formed MK party surged ahead to become the third-biggest in parliament.

EFF and MK are both part of a leftist opposition alliance formed after the election to counter the coalition government led by the long-ruling African National Congress (ANC) and the business-friendly Democratic Alliance.

But Shivambu's switch indicates the rising appeal of MK, which largely draws on Zuma's popularity in his home province of KwaZulu-Natal.

"This is a testing moment, ... the EFF will have to pass this test or it will have to die," Malema told reporters at a news conference.

Zuma was expelled from the ANC this year after he took the reins of MK, which did better than expected in the election and played a key role in bringing the ANC's share of the vote below 50%.

The EFF and MK have similar policies, including the nationalisation of banks and mines, and the expropriation of land for the benefit of Black farmers. They both rail against "white monopoly capital", which they say still controls South Africa's wealth three decades after the end of apartheid.