British travellers have been advised against visiting Zimbabwe in "all but exceptional" circumstances.
The Foreign Office said political tension was continuing to run high in the African nation, with results of the presidential election still not known.
Britons already in Zimbabwe were urged to keep a low profile, and avoid demonstrations and public transport.
The Foreign Office's previous advice against travel to Zimbabwe was limited to farming areas of the country.
Its latest advice said: "We advise against all but essential travel to Zimbabwe at this time due to the continuing tension surrounding the election and the deployment of uniformed forces (police and military) and war veterans across the country.
"The current situation is unpredictable, volatile and could deteriorate quickly, without warning""In the absence of officially confirmed results of the presidential election there continues to be a high level of political tension and uncertainty.
"The current situation is unpredictable, volatile and could deteriorate quickly, without warning."
Zimbabwe's opposition Movement for Democratic Change says its leader Morgan Tsvangirai won the election outright, but this is disputed by Robert Mugabe's ruling Zanu-PF party.
The Foreign Office advice added: "Farmers or agricultural workers who used to operate in Zimbabwe are advised that in the current tense environment it is dangerous to visit their former properties or other agricultural areas. A number of such visits have led to confrontation and violence.
"You should also avoid areas where war veterans are active."
On Wednesday, British Prime Minister Gordon Brown told the United Nations Security Council that "no-one thinks, having seen the result at the polling stations, that President Mugabe has won this election".
Meanwhile, South Africa has joined international calls for Zimbabwe's electoral commission to release the results of last month's election.
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