Former Bengal Chief Minister Buddhadeb Bhattacharya was admitted to a hospital in south Calcutta on Wednesday afternoon as his breathing-related problems aggravated and his health condition is stated to be very critical, officials said in the evening.

Bhattacharya, 76, has tested negative for Covid-19, they said.

He has been put on ventilator support at the critical care unit of the private hospital, the sources said.

"Mr Bhattacharya's RT-PCR test showed he was Covid negative and that is good news. The CT scan of his brain showed old lacunar infarcts. He is undergoing treatment at the CCU," a senior official of the hospital said.

"He is presently on non-invasive ventilation (BIPAP) and maintaining an oxygen saturation of 95 per cent. He remains critical but responding to current treatment," he added.

"Repeated tests showed an accumulation of carbon dioxide and a lower pH level, following which doctors decided to electively ventilate him after proper consent of family members," the offcial said.

"His condition is very critical," he said further.

Bhattacharya's pulse and blood pressure are stable and the veteran politician is on IV antibiotics and other supportive medications said a statement released by the hospital.

His oxygen saturation deteriorated to 70 per cent in the morning and he became unconscious, following which he was rushed to the hospital.

A two-member team of senior doctors was constituted to monitor the health condition of the veteran CPI(M) leader.

Bhattacharya, who was the chief minister of West Bengal from 2000 to 2011, has been suffering from COPD (chronic obstructive pulmonary disease) and other old age-related ailments for quite some time.

He had stepped down from the CPI(M)'s politburo, the central committee as well as the state secretariat in 2018.

Chief Minister Mamata Banerjee expressed her concern over Bhattacharya's health condition and prayed for his quick recovery.

"Concerned to hear that former Chief Minister Buddhadeb Bhattacharjee has been hospitalised with breathing problems. Praying for his speedy recovery and wishing him well," she said.

Governor Jagdeep Dhankhar also wished him a speedy recovery.

State Urban Development Minister Firhad Hakim visited the hospital and spoke to doctors.

After speaking to the doctors, Hakim briefed the chief minister about Bhattacharya's present health condition.

CPI(M) state secretary Surjya Kanta Mishra urged the people to cooperate with the hospital authorities.

"Comrade Buddhadeb Bhattacharya has been admitted for treatment of acute exacerbation of his respiratory problems in Woodland. No visitor is being allowed now. Well-wishers are requested to co-operate with the hospital authority for maintaining strict discipline in this regard," he tweeted.