Tsvangirai has been in and out of hospital since June 2016.
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.
Tsvangirai has been in and out of hospital since disclosing in June 2016 that he had colon cancer. He returned to a Johannesburg hospital for his latest round of treatment in early January
Family sources told local news outlets he is suffering from exhaustion, weight loss and muscle thinning.
Mr Tsvangirai became the symbol of resistance to long-time leader Robert Mugabe's ageing, repressive regime in the mid-2000s, leading the Movement for Democratic Change (MDC).
During his long political struggle against Mr Mugabe - who finally lost power in November last year - he has been badly beaten and imprisoned numerous times.
-Additional reporting done by BBC