Pondicherry Lt Governor K R Malkani died on Monday of cardiac arrest, official sources said. He was 82.
Malkani complained of chest pain at around 1700 IST and was immediately rushed to Government Hospital, Pondicherry.
Malkani, who was admitted to the intensive care unit, breathed his last at 1915 IST.
The Pondicherry Cabinet has condoled his death.
A team of doctors from JIPMER and Apollo hospitals, Chennai, tried in vain to revive Malkani.
Malkani was the 15th Lt Governor of Pondicherry and had assumed office on July 31, 2002. He was the second Lt Governor after R K Vyas to die in office.
As soon as the news of Malkani's admission into the ICU spread, the chief minister and his Cabinet colleagues, Assembly Speaker M D R Ramachandran and other political leaders rushed to the hospital.
Expressing shock and grief at the demise of Malkani, Prime Minister Atal Bihari Vajpayee said in his death, the country had lost an eminent nationalist thinker and a dedicated political personality.
Recalling his long association with Malkani in Bharatiya Jana Sangh and later in Bharatiya Janata Party, Vajpayee said that he combined his political activism with a life long commitment to research-based writing.
"He will be especially remembered for his book on the history of national movement in Sindh and his advocacy of Hindu-Muslim unity," he added.
Human Resources Development Minister Murli Manohar Joshi said Malkani's death was a personal loss to him.
"It is a great personal loss to me as we had been associated for long," he said in his condolence message.
"He was a man of great qualities and committed to human values. Throughout his life, he fought for betterment of downtrodden and the poor," Joshi said.
Rashtriya Swayamsevak Sangh ideologue and contemporary to top BJP leaders, including Vajpayee and Deputy Prime Minister Lal Kishenchand Advani, K R Malkani was one of the last members of the old guards of erstwhile Jan Sangh.
Born on November 19, 1921 in Hyderabad in Pakistan's Sindh Province, Malkani dominated the ideological battleground of BJP and its predecessor Jan Sangh.
A former editor of RSS mouthpiece Organiser between 1949 and 1983, Malkani served the Sangh Parivar faithfully for decades and became the Lt Governor of Pondicherry last year.
A teacher, a journalist and a researcher, he was associated with Congress and Students' Federation between 1939 and 1941 before joining the RSS of which he later became the Pracharak. He was also the editor of RSS newspaper Motherland from 1971 till it was closed in 1975.
Malkani, a member of Rajya Sabha in 1994, was detained under Maintanence of Internal Security Act during the Emergency.
By temperament, he always remained a teacher preaching discipline and carrying convictions on his sleeve. He never hesitated to speak up his mind often causing embarrassment to his party colleagues.
Malkani was a lecturer in DG National College in Hyderabad (Sindh) from 1945 to 1947 and Niemen Fellow at Harvard University (1961-62). He was also a member of several parliamentary committees.
Among his publications are The Midnight Knock (1977), The RSS Story, The Sindh Story and The Politics of Ayodhya and Hindu-Muslim Relations.
Starting his political career as a member of Jana Sangh, Malkani rose to become the vice-president of the BJP, the new version of Jan Sangh, in 1991.
Malkani is survived by two sons and a daughter. His wife died one-and-a-half years back.