Lawyer M. Ravi has filed a court challenge over a criminal defamation case involving Daniel De Costa, arguing that the charge faced by his client contravenes his constitutional right to equality before the law.

De Costa was charged in court last year over an article published on sociopolitical website The Online Citizen.

He had sent an e-mail in September that year that defamed members of the Cabinet.

He had intended that the e-mail's contents would be published on the site, court documents said.

Terry Xu, the site's editor, is also facing the same charge.

But Mr Ravi pointed out that at the material time, Prime Minister Lee Hsien Loong's siblings - Dr Lee Wei Ling and Mr Lee Hsien Yang - had made "similar or worse imputations" directed at PM Lee, a member of the Cabinet.

The prosecution of Xu and De Costa, but not the Lee siblings, is an infringement of the right to equality laid out in Article 12 of the Constitution, Mr Ravi argued.

Article 12 states that all persons are equal before the law and entitled to the equal protection of the law.

"The necessary corollary is that the law should be enforced equally," Mr Ravi said in his application to the court.

"If the statements of the Lee siblings are not prosecuted, (De Costa) should similarly not be prosecuted, a fortiori when the statements of the Lee siblings were more direct, and the allegations more severe."

The court hearing for the latest challenge is set for Jan 2 next year.