(CNN) Morgan Tsvangirai, Zimbabwe's longtime opposition leader and former prime minister, has died in a South African hospital at the age of 65, according to MDC party spokesman Nelson Chamisa.

Tsvangirai had been in and out of hospital since disclosing in June 2016 that he had colon cancer. He returned to Johannesburg for another round of treatment in early January.

Tsvangirai was the veteran leader of the Movement for Democratic Change, and one of the most prominent critics of the former Zimbabwean strongman Robert Mugabe and his ruling ZANU-PF party.

His death greatly increases the power of ZANU-PF ahead of elections later this year. Emmerson Mnangagwa, who took over as President and ZANU-PF leader after Mugabe's resignation last November, will be the heavy favorite to win the vote.

The African National Congress in Zimbabwe's southern neighbor, South Africa, sent its sympathies.

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