A former National Environment Agency (NEA) in-house lawyer became the 36th man in the online vice-ring case to be jailed for commercial sex with an underage girl.

Chia Kok Peng, 40, unemployed, was given 11 weeks' jail on Monday for engaging in paid sex with a 17-year-old escort.

He was among 51 men charged in court for commercial sex with the same minor in 2012. He was supposed to have gone on trial on Monday, but pleaded guilty.

The court heard that the minor was one of the escorts working for Tang Boon Thiew, 40, who started operating the online vice ring in 2010 on the viemodels.com website.

Customers who wished to engage the services of any of the escorts advertised on the website would proceed to contact Tang using the contact information.

The minor started working for Tang in September 2010. She was described as an 18-year-old polytechnic student.

Sometime on Oct 29 that year, Chia came across the profiles of the minor and another escort known as Angeline on the website. He contacted Tang via a mobile phone message and tried to book Angeline for sex but was later told that she was not available at Chia's preferred timing.

As such, Chia agreed to book the minor and pay $500 for her services.

At about 7.30pm that day, he booked a room at Hotel 81 Bencoolen and waited for the minor. She came and they had sex. Before she left, he paid her.

He could have been jailed for up to seven years and/or fined for commercial sex with a person under 18 years old.