"He was stable last (Saturday) evening," said Ajay Kumar Ray, director of the state-run SSKM hospital, where Justice (Retired) Karnan was admitted after he complained of chest pain.
A source from the Presidency Correctional Home said Justice (retired) Karnan was "still admitted at the hospital".
"He is under observation. I do not have any information about his illness or the kind of treatment he is getting," the source added.
The retired judge was lodged in Presidency Correctional Home on Wednesday after he was brought to Kolkata from Chennai following his arrest by the West Bengal CID officials from near Tamil Nadu's Coimbatore on June 20 in a case of contempt of court.
The controversial judge was sentenced to six-month imprisonment on May 9 by the Supreme Court and thereafter remained untraceable till his arrest.
Justice (retired) Karnan was held guilty of contempt for his utterances against the Chief Justice of India and other judges of the higher judiciary. Karnan who had left Kolkata for Chennai went underground thereafter. He retired from service earlier this month.