HARARE (Reuters) - Zimbabwe's High Court on Monday ruled that confessions by the chief prosecution witness implicating a key ally of Prime Minister Morgan Tsvangirai were invalid as they were not made freely.

Roy Bennett, a senior official of Tsvangirai's MDC party is on trial for terrorism and may face a death sentence if convicted. He, however, denies the charges and says he is being persecuted by President Robert Mugabe's ZANU-PF party.

Bennett, a white commercial farmer and Tsvangirai's nominee for deputy Agriculture Minister, was arrested on the day some of his MDC colleagues were sworn in as ministers last February.

His arrest and trial has raised tensions within a fragile power-sharing government formed by Mugabe and Tsvangirai last February following Zimbabwe's disputed elections in 2008.

"The witness gave evidence...with the greatest reluctance. His demeanour is that of a deeply aggrieved citizen with a gripe against the state," said Judge Chinembiri Bhunu of statements by state witness Peter Hitschmann linking Bennett to a terror plot.

Arms dealer and former police officer Hitschmann, 49, was jailed for three years in 2006 for possession of dangerous weapons, a conviction and sentence he is appealing, but was acquitted on the more serious terrorism charges.

The prosecution, led by attorney general Johannes Tomana, wants the High Court to convict Bennett by relying on written confessions and a video recording made by Hitschmann in 2006.

Hitschmann has said he was tortured into making the confessions during interrogation at a military barracks in March that year.

Bhunu, however, agreed with state lawyers that Hitschmann was a hostile witness and allowed the prosecution to cross-examine him in its bid to expose what it says were inconsistencies in his testimony.

"He views the state as an adversary...I have no option but to find that the witness is adverse to the state case," he said.

Mugabe has refused to appoint Bennett, saying the courts have to clear him first.

Click here to sign up to receive our free daily headline email newsletter