Zimbabwe's prime minister, Morgan Tsvangirai, announced that he was boycotting the country's troubled unity government today, citing the "persecution" of a top aide being tried on what are widely seen as trumped-up coup allegations.

At a news conference today, Tsvangirai said: "We are not really pulling out officially." But he added that his party would not attend cabinet meetings or engage in other executive work with President Robert Mugabe's Zanu-PF party. Tsvangirai said his Movement for Democratic Change would continue parliamentary activities.

Former white farmer Roy Bennett is being tried on charges linked to long-discredited allegations that his party, the MDC, plotted Mugabe's violent overthrow.

The MDC's move demonstrates deep unhappiness within the party with the coalition. But Tsvangirai has repeatedly said he sees the coalition as the only way to ensure Zimbabwe's future, and he made that clear again by stopping short of bringing down the government by pulling out altogether.

Tsvangirai and Mugabe entered the unity government in February after two violence-plagued elections left the country at a political standstill and in economic ruin. "Until confidence has been restored we can't continue to pretend that everything is well," Tsvangirai said. "It is our right to disengage from Zanu-PF."

Bennett, who was ordered back to jail earlier this week after seven months on bail, is due to stand trial on Monday.

Tsvangirai had nominated Bennett as deputy agriculture minister in the coalition. Bennett was arrested the day the cabinet was sworn in in February and charged with weapons violations. He denies the charges against him. "Roy Bennett is not being prosecuted, he is being persecuted," Tsvangirai said today.

The European Union said yesterday it was "deeply concerned" over Bennett's jailing. The EU added that it regretted "that politically motivated abuse persists in the country".

In Washington yesterday, US state department spokesman, Robert Wood, told reporters that the case against Bennett was a "blatant example of the absence of the rule of law in Zimbabwe".

Zimbabwe's neighbours had urged Mugabe, who has held power since independence in 1980, to form the partnership with Tsvangirai. In forming their coalition, the longtime opponents pledged to work together to reverse the country's economic and political collapse. Since the coalition was formed, Mugabe has demanded that Tsvangirai do more to get international sanctions lifted and restore foreign aid and investment. Tsvangirai has condemned continuing human rights violations.

The coalition is Mugabe's only hope of taking Zimbabwe out of international isolation, and it has brought Tsvangirai closer to power than any election.