92-year-old Nathan suffered a stroke early this morning and is in critical condition in the Intensive Care Unit at Singapore general hospital, the Prime Minister's Office said in a statement.
Nathan was Singapore's sixth and longest-serving president, had earlier suffered a stroke in April last year.
Following that his family said he was recovering and undergoing therapy.
Nathan was in office for two terms from 1999 to 2011 and officially stepped down as President on August 31, 2011 after announcing that he would not seek a third term in office. He was succeeded by President Tony Tan Keng Yam.
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Prior to becoming President, he held key positions in the civil service, in security, intelligence and foreign affairs.
He was appointed as Singapore's High Commissioner to Malaysia in 1988 and later Singapore's Ambassador to the United States of America from 1990 to 1996.
He also served as Singapore's Ambassador-at-Large, and later pro-chancellor of the National University of Singapore.
He is also a recipient of Pravasi Bharatiya Samman, the highest honour accorded to the Indian-origins oversees by the Indian government in 2012.