The 95-year-old leader has been kept under observation, the doctors said.
"Basu has been closely examined and found to have a little pneumonia," the doctors said after a 5-member medical board started treatment of the former Chief Minister and conducted CT-scan, X-ray and blood tests.
A medical bulletin said Basu was admitted with chest congestion and infection and thorough examination detected he had been afflicted with slight pneumonia.
Prime Minister Manmohan Singh rang up West Bengal Chief Minister Buddhadeb Bhattacharjee this evening to enquire about Basu's health, CPI(M) sources said.
Before being taken to hospital, Basu was examined at his Indira Bhavan residence in Salt Lake area by a four-member team of doctors headed by his personal physician A K Maity who decided to admit him to AMRI hospital for a thorough treatment for his bronchial congestion.
Basu's personal aide Joykrishna Ghosh said that the Marxist veteran's other health parameters were okay.
One of the doctors, Trishit Roy said Basu's condition was stable. Roy said the doctors considered it was 'high time' Basu was shifted to hospital for a thorough treatment of his chest infection.
The last time Basu had been admitted to a hospital was on July 12 after he fainted at his residence following breathing trouble.
CPI(M) politburo member and state secretary Biman Bose visited the hospital after hearing Basu's admission.
State Finance Minister Asim Dasgupta and Lok Sabha's former Speaker Somnath Chatterjee also went to the hospsital.
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