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Susan Tsvangirai was killed instantly and the Prime Minister is said to be in a stable condition in hospital.
The lorry driver has admitted falling asleep at the wheel, according to reports.
A source in his party said: "I can confirm that Susan Tsvangirai has died following the accident."
Sky News
Africa correspondent Emma Hurd said the accident happened as the couple were travelling to a rally south of the capital Harare.
"The car they were travelling in was sideswiped by a lorry," she said.
Robert Mugabe is reported to have visited Mr Tsvangirai in hospital.
State television reported nothing about the accident in its two evening news bulletins.
The Foreign Office expressed "deep sadness" at the news.
A spokeswoman said the UK Government offered its condolences and was tracking developments closely.
Tsvangirai was sworn in last month as prime minister of a government with old rival President Robert Mugabe.
It was set up to try to end a political and
economic crisis that has brought the once relatively prosperous country to ruin.
Tsvangirai, who turns 57 on Tuesday, has been married to Susan for 31 years. They have six children.
Although Susan was not actively involved in Tsvangirai's party, she has supported him and appeared at campaign rallies with her husband over the last 10 years.
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