The love that Ms Darinne Ko and he shared was "forbidden" and "frowned upon by society", said law professor Tey Tsun Hang.

Giving his account of the relationship he had with his former student for the first time, he painted a picture of them as star-crossed lovers.

Taking the witness stand in the sex-for-grades corruption trial on Monday, he pointed to a letter written by Ms Ko, now 23, before their relationship ended to show how "intense" it had been. "She wrote this message of love lost, hope distressed, togetherness and departure," he said.

While giving his version of events of their relationship, Tey sounded wistful at times and at other points, was inaudible - leading Chief District Judge Tan Siong Thye to ask him to speak up.

"The entire relationship, how it started, became intense and how it ended had never anything to do with money or with any notion of exploitation, abuse or any wishful notion of taking advantage of anybody, let alone this idea of corrupt relationship," Tey said.

The 41-year old former district judge explained how a chequebook entry for $2,500 dated July 2, 2010 came about. He said it was payment for Ms Ko's gifts of a Montblanc pen, two tailored shirts, an iPod Touch and an Italian dinner he hosted at Garibaldi - which he already knew the cost of having been pre-arranged weeks before it actually occurred. He did not go into detail as to why it was unencashed.

These items are the very subject matter of four of the six charges of corruption Tey faces. The other two charges are relating to sexual intercourse with Ms Ko.

The prosecution had earlier closed its case, and Tey is the first defence witness.