The 65-year-old 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 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 said.
Tsvangirai's illness has divided his Movement for Democratic Change (MDC) party, with officials publicly jockeying to succeed the former trade union leader.
Last month, Tsvangirai said it was time for the older generation to step back and make way for "new hands," raising prospects of leadership change.
Without its founder at the helm, the MDC is likely to face immediate instability and could even split, handing a gift to new President Emmerson Mnangagwa in an election expected within 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 spokesman Luke Tamborinyoka said the MDC leader was "stable but the nation should keep on praying."