FILE PHOTO: Zimbabwe's President Robert Mugabe (front R) and Prime Minister Morgan Tsvangirai (L) attend a conference of parties and civic society groups reviewing a draft constitution that, if adopted, will lead to Zimbabwe's next election, at a hotel in Harare October 22, 2012. REUTERS/Philimon Bulawayo/File Photo

HARARE (Reuters) - Zimbabwe’s main opposition leader, Morgan Tsvangirai, is critically ill in a South African hospital and his supporters should “brace for the worst”, a party source with knowledge of his condition said on Tuesday.

Without its founder at the helm, Tsvangirai’s Movement for Democratic Change (MDC) is likely to face immediate instability and could even split, handing a gift to new President Emmerson Mnangagwa in an election expected in the next six months.

Mnangagwa came to power in November after a de facto military coup against 93-year-old Robert Mugabe, the former guerrilla leader who had run Zimbabwe since independence from Britain in 1980.

Tsvangirai, 65, has been in and out of hospital since disclosing in June 2016 that he had colon cancer. He returned to Johannesburg in neighboring South Africa for his latest round of treatment in early January.

“From the medical report that I received yesterday the situation is not looking good. He is critically ill and we should brace for the worst,” the source in Tsvangirai’s MDC said.

Tsvangirai’s Twitter feed contested the report, saying the former union leader who emerged two decades ago as Mugabe’s principal political challenger was “shocked” to read of his deteriorating condition.

“Of course I have cancer and not feeling too well but I am stable and the process is under control. I have been frequently on Twitter of late, I am recovering,” the feed said.

Tsvangirai spokesman Luke Tamborinyoka said the MDC leader was “stable but the nation should keep on praying”.

Tsvangirai’s illness has exposed deep rifts in the MDC, with his subordinates jockeying publicly to succeed him. Last month, Tsvangirai said it was time for the older generation to step back and make way for “new hands”.