Testifying to a parliamentary committee, Brown made clear Tsvangirai’s appointment as prime minister would not lead to an immediate change in relations with Zimbabwe that might open the way to large-scale aid to help rebuild its ravaged economy.
Tsvangirai was sworn in on Wednesday by President Robert Mugabe following months of wrangling since they agreed last September to share power.
Brown said he feared Mugabe, who has ruled Zimbabwe since independence from Britain in 1980, would still block change.
The British prime minister said he had told Tsvangirai on Tuesday Britain wanted to see humanitarian aid getting to people in distress, such as those affected by a cholera outbreak.
"I also said to him that until the government of Zimbabwe could convince us that there were going to be free and fair elections and … the removal of repressive legislation and clearly the release of political prisoners, until these things happened, we could not treat Zimbabwe as if it was an ordinary country," Brown said.
"I hope there will be considerable pressure by the international community to release political prisoners, to get in a credible team to deal with the finances (and) to have a clear roadmap to the next election," he said.
Britain has been one of the fiercest critics of Mugabe, accusing him of destroying the country’s economy and using militias to suppress opposition.
Mugabe’s government in turn blames Britain and other Western nations for Zimbabwe’s meltdown.
The country is suffering unemployment above 90 percent, prices double every day, half the 12 million population need food aid and a cholera epidemic has killed nearly 3,500 people.