Sources at the private hospital where Mitra was admitted to said he died due to heart and age-related ailments.
"He was admitted to the hospital a few days back after his creatinine level was found to be high during a routine check-up. He was a patient of chronic obstructive pulmonary disease (COPD) and also had other age-related ailments," a senior official of the hospital said.
Mitra passed away around 1.30 am following a cardiac arrest, the hospital sources said, adding he had tested negative for Covid-19. He is survived by his wife and son.
According to a family member of the Congress leader, he was taken to the hospital a few days back for a regular health check-up. Mitra had undergone a bypass surgery some years back when he was a Lok Sabha MP.
Gaurav Gogoi, Congress MP and All India Congress Committee (AICC) in charge of party affairs in the state condoled the demise of the West Bengal Pradesh Congress Committee (WBPCC) chief. "My heart goes out to the family of the Lt Somen Mitra. He was a giant of Bengal, and he touched the lives of millions of people in his long journey. My condolences to his family and all those who admired him. His legacy will not be forgotten," Gogoi tweeted.
Known as a bte noire of West Bengal Chief Minister Mamata Banerjee, it was during Mitra's second term as Congress president in the late '90s that Congress lost its status as the principal opposition of the then mighty Left Front regime to the Trinamool Congress (TMC). Born on December 31, 1941, in Jessore district of erstwhile East Bengal (now Bangladesh), Mitra was the eldest of five siblings.
A stalwart in Bengal politics, Mitra's political career began during the tumultuous '60s as a student leader and spanned over five decades.
After being baptized in politics as a student leader in 1967, when Bengal had its first non-Congress government, Mitra, through his organisational and oratory skills, quickly rose through the ranks and became one of the most popular leaders of the party along with late union minister Priya Ranjan Dasmunsi.
Mentored by Congress stalwarts such as A B A Ghani Khan Chowdhury, Mitra's first brush with electoral politics came in 1972 when he became the youngest MLA in the West Bengal Assembly from the Sealdah seat at the age of 26.
Except 1977, Mitra continued to win the Sealdah assembly segment, which now ceases to exist after delimitation, for six consecutive terms from 1982-2006.
Commonly known as 'Chhorada' (younger brother), Mitra was one of the most firebrand politicians of the 1960s and '70s and played a crucial role in the fight against the Naxals in Kolkata during that period.
He was considered as "favourite" of the Congress high command, who enjoyed an excellent rapport with the Gandhi family. But that did not stop him from defeating Congress president Sonia Gandhi's handpicked candidate D P Roy during the 2000 Rajya Sabha poll by pitting a rival candidate.
Mitra, who went on to become three-time president of the Congress' West Bengal unit from 1992-1996, 1996-1998 and then again from September 2018, was instrumental in clocking the best tally of 82 seats against the Left Front in the 1996 assembly polls.