Zimbabwe's high court on Monday struck out evidence from a key witness in the treason trial of a top aide to Premier Morgan Tsvangirai, after the man said he had been tortured into testifying.

Roy Bennett, the treasurer of Tsvangirai's party, is accused of plotting to assassinate President Robert Mugabe in 2006 in a conspiracy already dismissed by the courts in an earlier case.

Peter Hitschmann was initially charged with Bennett but later was convicted of a lesser charge of illegal possession of firearms in 2006.

He had been called as the main witness to testify against Bennett, but in court he accused security forces of torturing him while in custody to extract a statement.

Justice Chinembiri Bhunu said statements made by Hitschmann could not be used as evidence since they were not signed and Hitschmann said his interrogators used force to make him implicate Bennett.

"He said he involuntarily made the statements under torture. The statements were obviously inadmissible and not used at his own trial," Bhunu said.

"It's common cause that if they were inadmissible at his own trial they are equally inadmissible in this case."

Bennett was Tsvangirai's pick for deputy agriculture minister in the power-sharing government with President Robert Mugabe, formed nearly one year ago.

He is accused of providing Hitschmann with funds to buy weapons to be used to topple Mugabe. His prosecution is one of the issues hampering the work of the power-sharing government.