All six statements, which law professor Tey Tsun Hang made to Corrupt Practices Investigation Bureau (CPIB) officers during questioning last year, will be admitted as evidence in his sex-for-grades corruption trial.

Chief District Judge Tan Siong Thye on Tuesday, decided after the trial within a trial, that he was "unable to believe" Tey's allegations that the statements made in April and May 2012 were done involuntarily.

He said the CPIB officers were "truthful witnesses" but Tey, on the other hand, had "exaggerated and fabricated" some of the details on the day of his arrest, which may have affected his psychiatric diagnosis of altered mental status.

The judge said he also found Tey's several voluntary visits to the CPIB "puzzling", especially on occasions when he claimed to be unwell and could have rested at home.

The chief district judge said that if Tey had found CPIB officers' acts so "heinous", he could have reported these to the police but no such reports or complaints were made.

"Given the accused's extensive legal training and background, it is difficult to believe he was coerced,'" he added.

Chief District Judge Tan's ruling comes after a trial within a trial, held to determine if the six statements Tey, 41, made to CPIB officers were voluntary and can be admitted as evidence.

The main trial, in which Tey has been accused of obtaining gifts and sex from his 23-year old former student in return for giving her better grades, resumes after this.