The Workers' Party's (WP) potential candidate, National University of Singapore sociologist Daniel Goh, has denied allegations of having had an affair with a former student.

In the wake of what he described as a poison pen e-mail to the press, Dr Goh, last night, in a text message to The Straits Times, and a Facebook post later, said: "I categorically refute the baseless allegations and I question the timing of the poison pen letter, coming immediately after the candidate introduction.

"I have been in the public eye, involved with WP for over two years. If the intention is to alert the party to alleged moral failings, it would have been done earlier, and directly to the party leaders. I have always maintained a professional relationship with my students. This is very hurtful, and unfair to my family."

Dr Goh, 42, is married to a housewife, 39. They have a three-year-old son. He was introduced on Wednesday as a WP candidate for the coming polls and may be fielded by the party in East Coast GRC.

A letter sent by a "Max Chan" to several media organisations last night alleged that Dr Goh had an affair in 2008 with an NUS postgraduate student, and questioned Dr Goh's character and morals.

The letter also alleged that the student's boyfriend, whom he referred to as "J", found out about the alleged affair through her phone messages.The e-mail was also sent to WP secretary-general Low Thia Khiang and chairman Sylvia Lim.

Dr Goh said he advised the student for her master's thesis from 2007 to 2009. She then left Singapore to get a PhD, and returned to be a teaching assistant but left the post in 2012.

He said they still keep in touch "because she respects me a lot as I advised her on her PhD studies in the US". He did not know who wrote the letter.