The man, who cannot be named, would be at risk of harm if he were returned to President Robert Mugabe's regime, the Asylum and Immigration Tribunal said.
The decision will force the Government to rethink its deportations policy to the southern African country.
Campaigners have successfully claimed that people returned from the UK are regarded as "spies" and "traitors" by the Mugabe regime.
The tribunal chairman Mark Ockelton said: "He has a well-founded fear of persecution ... if he is returned to Zimbabwe."
Although the failed asylum seeker had been "fraudulent" and " deliberately dishonest" in his dealings with the British authorities, the fact that he had spent time in the UK would put him at risk at home, the tribunal went on.
"The fact that the appellant made a false claim, so generating the risk which would otherwise not have existed, does not alter the fact that the real risk of serious harm exists now," the ruling went on.
The tribunal criticised the Home Secretary, Charles Clarke, for his department's research into conditions in Zimbabwe and for the lack of evidence uncovered by a fact-finding delegation sent by the Government in September.
Evidence from the Home Secretary appeared to show that deportees were escorted on planes where their papers were handed over by UK officials to the air crew.
"At that point, it appeared to us that the respondent (the Home Secretary) ceased to have any very clear interest in what happened.
"We find the respondent's lack of interest in the process by which individuals that he returns to Zimbabwe are received by the Zimbabwean authorities rather alarming."
The British Government delegation to investigate conditions in Zimbabwe had been made up of civil servants involved with policy matters, Mr Ockelton said.
"The way in which the investigation was conducted, and the way in which the results were presented to us, gives rise to the possibility - we say no more than that - that the investigators may have had existing policy in mind rather more than the discovery of new facts.
"Despite the facilities available to the investigation and the level at which it was conducted, it reveals nothing of the actual process which returned asylum-seekers go through on their arrival at Harare Airport."
After the hearing, the Refugee Council's Tim Finch said: "We're delighted. It's a very clear ruling and its implication is clear for all the other failed asylum seekers from Zimbabwe.
"It is a credit to those 140-odd Zimbabweans who staged a hunger strike while in detention this summer, who forced this issue to the top of the agenda.
"They can now feel reasonably secure.
"This judgment shows that although they have no status as failed asylum seekers they cannot be removed."
Mr Finch added: "The judge could not have been more dismissive of the way the Government delegation conducted its work in Zimbabwe earlier this year.
"The fact-finding mission just blew up in the Government's face.
"He made some devastating comments about the cavalier way in which the Government treat failed asylum seekers by putting them on the plane, withholding their documents and not really caring what happens to them at the other end.
"The outcome is that failed asylum seekers from Zimbabwe end up being questioned by Mugabe's security forces, who are deeply nasty people."
Mr Finch said that today's ruling directly affected the 140-plus Zimbabweans who had been detained this summer.
But, as Mr Ockelton said in his judgment, the Government has refused to disclose the full number of Zimbabweans who are facing deportation.
Failed Zimbabwean asylum seeker Noble Sibanda, 29, said: "We are very, very happy. We can now get a safe haven."
Mr Sibanda fled his home in Bulawayo because he was in danger from the Mugabe regime after his involvement with opposition party Liberty and is now part of campaign group United Network of Detained Zimbabweans.
"All the evidence put forward to the judges showed what it's like when people are wrongfully sent back to Harare," he said outside the hearing in central London.
"All we're asking for is sanctuary and time to regroup so that we can take care of the mess in our country.
"We hope Britain will now offer that support and that this will be an example to other countries around the world where our fellow Zimbabweans have sought asylum, such as Australia."
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