"Condition of Comrade Bardhan has worsened today. He was put off ventilator yesterday and was able to breath normally. But today, his blood pressure (level) fell and his condition has turned very critical now," CPI national secretary D Raja said.
Raja added that doctors at the G B Pant hospital, where the 92-year-old leader is being treated since December 7, are making "best efforts" to see improvement in Bardhan's health.
He later rose to become the General Secretary and then President of All India Trade Union Congress, the oldest trade union in India.
Bardhan had moved to Delhi politics in the 1990s and became the Deputy General Secretary of CPI. He succeeded Indrajit Gupta as the General Secretary of the party in 1996.