Roy Bennett, of the Movement for Democratic Change (MDC), missed the swearing-in ceremony for deputy ministers last month because he was in jail on terrorism charges. Mr Bennett, 51, was released on 12 March on £3,400 bail.
He was sworn in as a senator almost immediately, and the MDC expected him to be sworn in as deputy agriculture minister last week. But Mugabe "has refused point-blank", an official said yesterday.
"He says he won’t swear in Bennett even if the MDC takes the issue to the South African Development Community," the official said. The president is said to claim that Mr Bennett faces "very serious charges".
Mr Bennett’s lawyers say the charges – which relate to a long-discredited "assassination plot" – are fabricated.
Analysts say the nomination of Mr Bennett by the MDC leader, Morgan Tsvangirai, is confrontational. He would be serving under agriculture minister Joseph Made, who oversaw Mugabe’s land seizure programme and sanctioned the takeover of Mr Bennett’s coffee farm.
During Mr Bennett’s time as an MP, he pushed a Mugabe ally, justice minister Patrick Chinamasa, during a heated parliamentary debate. That led to him spending eight months in jail.
Mr Bennett believes the justice minister ordered his arrest last month.