Mitra, lodged in Alipore jail, was taken to state government-run SSKM hospital in the night, hospital officials said.
He complained of chest pain and breathlessness.
Arrested by CBI in December 2014, the former state Transport and Sports Minister is contesting polls from his constituency of Kamarhati on the outskirts of the city where polling was held yesterday in the fourth phase of assembly elections.
On Sunday also, Mitra had complaints of chest pain.