Supporters of Morgan Tsvangirai, Zimbabwe's opposition leader, are "bracing for the worst", Reuters news agency has been told.
Mr Tsvangirai, 65, has been in South Africa receiving treatment for colon cancer since January.
However, a source close to the politician has told Reuters "the situation is not looking good".
They added: "He is critically ill and we should brace for the worst."
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.
