Former communist allies have attacked India's government over a controversial civilian nuclear deal with the US.
Earlier, they formally withdrew support for the government after it vowed to press ahead with the agreement.
Communists say the accord could open Indian foreign policy to too much US influence. The government says it is needed to meet soaring energy demands.
US President George W Bush has spoken again of the importance of the deal, in talks with Indian PM Manmohan Singh.
The two men met on the sidelines of the G8 gathering in Japan. Afterwards, Mr Singh said relations between his country and America had never been so strong.
Mr Singh says he does not think his government is threatened by the withdrawal of communist support.
'Crisis'
Communist leader Prakash Karat said the government had "disregarded" parliament and "not been transparent" in going ahead with the "notorious" nuclear deal.
"The government says that it cannot place the text [of a nuclear safeguards agreement with the International Atomic Energy Agency (IAEA)] to the committee we had formed with them over the deal because it is classified," he said.
"We would like to know who has declared it classified? The government or the IAEA? I would like a clarification."
Mr Karat said the government had "plunged the country into a political crisis when it [is facing] double-digit inflation".
He said the communists would soon make public their notes and correspondence with the government over the nuclear deal.
Earlier, Mr Karat told reporters that the four left-wing parties had handed over two letters to President Pratibha Patil.
"We have submitted letters withdrawing support to the government. And we have requested her to quickly convene a session of parliament so that the [ruling party] can face a vote of confidence [to prove its majority]," he said.
Separately, veteran communist leader Jyoti Basu has expressed reservations about the left withdrawing support for the government.
"I want our comrades to protest against the deal and the way the Congress is pushing for it, but I don't want them to vote the government out of power," he told reporters.
"That will help the Hindu fundamentalist forces to gain hugely," he said.
The communists have 59 members in India's lower house of parliament.
The Congress-led government is hoping that a regional party will help them survive a vote of confidence and fend off early elections.
The Samajwadi Party has been a traditional political foe of Congress, but has said that its 39 MPs will support the governing coalition on the nuclear deal issue.
Reports say that the vote of confidence could be held later this month, ahead of the planned opening of the new session of the lower house of parliament on 11 August.
Off-limits
India is under pressure from Washington to sign the accord before the US presidential elections in November.
TIMETABLE FOR NUCLEAR ACCORD
Under the terms of the accord, India would get access to US civilian nuclear technology and fuel.
In return, Delhi would open its civilian nuclear facilities to inspection - but its nuclear weapons sites would remain off-limits.
Analysts say with the left pulling out of the governing coalition, the government only has 226 members in the 543-seat parliament, a good 46 seats behind the majority mark.
The deal now needs to be approved by the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), as well as by the 45-nation Nuclear Suppliers Group, which regulates global civilian nuclear trade.
Then it has to be presented to the US Congress for final approval.
Critics of the accord say it sends the wrong message as India has not signed the nuclear Non-Proliferation Treaty.
Mr Singh has said he will submit an application to the IAEA as soon as possible.
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