Former president K R Narayanan was in a "very critical" condition after he slipped into coma at the Army's Research and Referral Hospital in New Delhi Wednesday.

Narayanan has acute artery blockages and his condition is "very critical". He has slipped into coma, a medical bulletin issued by the hospital, said.

Eighty-five-year-old Narayanan was admitted to the hospital on October 29 after he complained of acute breathlessness and has since been on ventilator.

"Medical evaluation has revealed evidences of right side pnumonitis (pneumonia)," the bulletin said.