Pressure mounted on Manmohan Singh, India’s Prime Minister, to renegotiate a landmark nuclear deal with the United States when a key coalition partner threatened last night to bring down the Government in protest.

A.B. Bardhan, General Secretary of the Communist Party of India, said the “Left Front” that has backed the Congress Party’s ruling coalition since 2004, would withdraw its support if the deal was not changed.

“It seems to me to be inevitable,” he said in an interview with a private news channel. “The honeymoon … is over.”

Analysts said his comments were a negotiating tactic, since the Left Front was itself divided, but that there was a real risk that it would scale down its support for Mr Singh, making it harder to implement economic reforms.

India and the US, whose relations were chilly throughout the Cold War, finalised a historic agreement last month giving India access to American nuclear supplies for the first time in 30 years. The deal is the centrepiece of a new strategic relationship between Delhi and Washington, which sees India as a bulwark of democracy in Asia and a counterbalance to the military might of China.

However, after publishing its details this month, the two sides are still arguing over the fundamental issue of whether the deal allows India to carry out nuclear tests. And the agreement has been rejected emphatically by the Hindu nationalist Bharatiya Janata Party and by the Left Front, which both accuse the Indian Government of making too many compromises.

Last week, they heckled Mr Singh in Parliament as he read out details of the “123” agreement, which still requires the approval of the US Congress and international bodies.He responded by challenging the Left Front to withdraw its support from the government.

Mr Bardhan, for one, now seems to be calling his bluff. “I and the people of this country would like a stable Government," he said. "But stability doesn’t mean that the Government should take the country towards a military strategic partnership with the US.”

“I don’t want the Government to fall only on the nuclear issue; I want to clarify that. It has just capped all the other issues that have been going on.”

Mr Singh had already been facing criticism from the Left Front for failing to extend the benefits of India’s economic boom to the poor and needy. He needs the Left Front’s support because the ruling coalition, led by the Congress Party, does not control an outright majority of seats in Parliament.

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