He is now in the prison ward of Bangabandhu Sheikh Mujib Medical University (BSMMU) on charges of hurting religious sentiments.
He was shifted to Coronary Care Unit (CCU) from the cabin after he suffered the cardiac arrest around 9:30am on Saturday, BSMMU Director Abdul Majid Bhuyian said.
The 77-year-old was also suffering from other geriatric complexities but did not lose his sense, Bhuiyan added.
He was taken to the hospital from Dhaka Central Jail on Dec 6 last year after he complained of chest pain.
A former presidium member of the ruling Awami League, Siddique has been in prison since Nov 25, after he turned himself over to Dhanmondi Police.
He did not plea for bail.
He stands accused of hurting sentiments of Muslims in a flurry of cases filed across the country following his anti-Hajj remark during a discussion in New York on Sep 28last year.
He was eased out of the Hasina cabinet and the Awami League while he was abroad.
He returned home suddenly on Nov 23 amid protests and demands for his arrest over the remark.
A 1971 war veteran, Siddique still represents his constituency, Tangail-4, in Parliament.