CPI, citing Parliamentary procedures, asked the government to prove its majority after 19-member Trinamool Congress withdrew support.
Congress, however, said it has not lost all hopes on ties with Trinamool Congress, saying it still considers the party a "valuable ally".
"We have always considered Trinamool Congress our valuable ally. Even after all that Mamata Banerjee has said, we still consider her our valuable ally till a final result comes out," Congress general secretary Janardan Dwivedi told reporters here.
He said Congress party "will definitely discuss the issues raised by Banerjee with the government".
JD-U chief and NDA Convenor Sharad Yadav hailed Banerjee's decision, saying she had taken a "big step" to save the nation.
"I salute Mamata Banerjee for the step she has taken. She has taken a big step to save the nation," Yadav told PTI.
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He said "such an injustice was never done to people in last 64 years since the country got independence. By bringing FDI in multi-brand retail, the government is shutting the door on the employment of 22 crore people".
BJP said the decision of the Trinamool was the beginning of the "downfall of the UPA government".
"Practically it is now a Congress government... After the withdrawal of support by the Trinamool Congress, a serious question mark is there on the stability of this government," BJP chief spokesperson Ravi Shankar Prasad said.
"We need to applaud and compliment Mamataji for taking such a bold stand, purely in the interest of the people of the country," he said.
Samajwadi Party, a key outside supporter of UPA, blamed the Congress-led government's attitude for being behind the situation.
"The Congress government is behaving as if it enjoys complete majority. It is taking decisions without consulting allies," SP General Secretary Ram Gopal Yadav said.
CPI leader Gurudas Dasgupta said since 19 members have moved away from the government, "according to all principles of parliamentary democracy, government should prove its majority on the floor of the House".