By Subir Bhaumik
BBC News, Calcutta
Jyoti Basu won consecutive elections in West Bengal
The health of veteran Indian communist leader Jyoti Basu worsened on Wednesday, doctors attending him in a Calcutta hospital have told the BBC.
"He has been put on a full ventilator because he was not able to breathe," a hospital spokesman said.
"He has serious chest congestion and we have not been to clear it."
The 95-year-old is one of India's best known and most respected communist leaders whose political career has spanned decades.
'Praying for recovery'
"Drugs have not worked since his condition deteriorated late on Tuesday night," the spokesman said.
The left has lost seats in recent West Bengal elections
Mr Basu's long time personal aide Joy Krishna Ghosh said he remained confident the veteran Marxist would stage a full recovery, warning journalists not to spread false rumours about his health.
Mr Basu, who was chief minister of West Bengal state for more than 20 years, was admitted to the hospital with pneumonia on 1 January.
Thousands of people - including politicians of all shades, leading artists, writers and cultural personalities - visited the AMRI hospital where he is being treated on Wednesday.
"We are all praying for his recovery, but he is very frail," said Railway Minister Mamata Banerji, a long-time adversary of Mr Basu but one who respects him personally.
Ms Banerji visited the ailing leader around the same time as the West Bengal chief minister Buddhadev Bhattacharya.
Jyoti Basu was born into a well-off middle class family in what is now Bangladesh in 1914.
He was educated in Catholic schools and colleges before training as a barrister in London.
For more than two decades he led West Bengal's communist government.
Mr Basu retired on grounds of ill health in 2000, paving the way for his protege, Mr Bhattacharya, to take over as chief minister.
Analysts credit Mr Basu with restoring law and order to West Bengal after the turbulence of the 1970s, when several coalition governments were toppled and the state faced a bloodbath during the country's first Maoist rebellion.
His left coalition government is credited with land reforms and the introduction of village self-government that helped his CPI(M) party build a formidable power base in rural Bengal.