Lawyer and ex-DPP Spencer Gwee was on Friday found guilty of paid sex with an underaged prostitute.

Judge Toh Yung Cheong said the prosecution had proven its case beyond reasonable doubt, and found the 61-year-old guilty of one charge of having paid the Vietnamese prostitute $300 for sex at Four Chain View Hotel in Geylang on the evening of July 19, 2011.

In his written grounds, the judge addressed several key areas of Gwee's defence, including his claim that he did not speak Vietnamese and never had a conversation with the girl, even though there were telco records of SMSes exchanged between their phones.

"The communication between (the girl) and the accused was more or less limited to requesting sexual intercourse; (she) was not claiming that the accused recited the Iliad or the Odyssey," said Judge Toh.

"It was not out of the realm of possibility that the accused may have learnt a few Vietnamese phrases from his frequent visits to Four Chain View Hotel, pubs in Geylang, as well as his trips to Vietnam."

The judge also addressed an alibi that Gwee claimed to have, which was that he had been discussing business with a client around the time the paid sex occurred.

"There was a significant likelihood that (the client) was mistaken about the date or time he met the accused," wrote Judge Toh. "(And) even though (his) evidence would have been critical, his existence was never mentioned to the police during investigations by the accused."

Instead, the defence witness' evidence was contradicted by "objective, independent evidence" from the hotel, including both oral testimony and records that placed Gwee at the hotel at the time of the crime.

Sentencing for the case will take place next Friday.