Neither the prosecution nor the defence wanted to reveal the witness's identity.
In the Garage case, independent member of parliament Branimir Glavas and Krunoslav Fehir are suspected of having committed war crimes against local Serbs in the eastern city of Osijek in 1991.
After the last ten witnesses for the defence were questioned at the Zagreb County Court on Tuesday, a spokesman for the court, Kresimir Devcic, said that the motion by the prosecution to question one more prosecution witness was upheld because of the explanation that "the proposed witness possesses relevant knowledge".
His testimony is scheduled for Thursday, but Glavas's lawyer Ante Madunic said today that the defence team was against it.
Spokesman Devcic said this witnesses had not been interviewed in the probe so far, but lawyer Madunic claimed the opposite.
"That witness has changed his mind for reasons unknown to me and now claims that he will say the truth this time after he lied to the court previously," Madunic said, adding that the witness in question was a convict serving a 13-year term for criminal offences.
Madunic believes that the prosecution witness might ask for some concessions in exchange for his testimony.
The investigation in the Garage case resumed on Monday, after it was suspended for several weeks due to the poor health of suspect Glavas who had been on a 36-day hunger strike in November and December 2006.
Last week, the prosecution asked Investigating Judge Zdenko Posavec to order an additional explanation from psychiatrist Vera Folnegovic Smalc as it holds that her latest assessment of Glavas's condition was incomplete.
The psychiatrist said the patient's mental condition had improved, but suggested the continuation of psychiatric treatment. She recommended that Glavas should avoid stress due to his emotional lability, which means that he should not attend investigative hearings during the probe.
The prosecution insists that Dr Folnegovic Smalc should establish his concrete diagnosis, because in case of the issuance of an indictment, Glavas, in his capacity as an indictee, would be required by law to attend hearings during a possible trial.
Another investigation against Glavas, in the so-called Sellotape case, resumed last Wednesday before the Osijek County Court after it was suspended on 20 December when Glavas was declared unfit to follow the proceedings.
Glavas and another six people are suspected of involvement in war crimes against several Serb civilians by the Drava River. In the Sellotape case, 10 victims were killed and one who survived the execution now lives in Serbia.