Foreign Secretary David Miliband said: "The new government in Zimbabwe has made some notable progress since its inception in February.
"This makes news of the return to custody of 18 high-profile political detainees all the more disappointing," he added in a statement.
Miliband reiterated that the release of all political detainees was "one of the principle conditions for full international re-engagement with Zimbabwe".
He said that in a meeting in London last week with Zimbabwe’s new finance minister Tendai Biti, he had welcomed the "credible start" to the new government’s reform efforts.
"The return to custody of political detainees further highlights the reform needed in the judicial sector," Miliband said.
He said Britain and its international partners "continue to monitor the situation closely on the ground and continue to maintain close dialogue to encourage progress on reform".
Zimbabwe Prime Minister Morgan Tsvangirai’s party has warned that the detention of the activists threatens the survival of the country’s fledgling unity government.
Prominent rights campaigner Jestina Mukoko and 17 others were returned to custody on Tuesday, just two months after they were freed on bail over an alleged plot to overthrow President Robert Mugabe. – AFP