Mitra was brought to the hospital from Alipore Central Jail after he complained of chest pain and respiratory distress, SSKM Hospital sources said.
He was admitted to the cardiology department of the hospital, they said.
"As per version of the on-duty medical officer of the emergency department, Mitra complained of chest pain and respiratory distress," the sources said.
Mitra was arrested on December 12 by the CBI and was in its remand till today.
The additional chief judicial magistrate of Alipore Court remanded Mitra to judicial custody till January 2, following which he was taken to the jail.