"I told them (Left parties) that it is not possible to renegotiate the deal. It is an honourable deal, the Cabinet has approved it, we cannot go back on it. I told them to do whatever they want to do, if they want to withdraw support, so be it...," he told a Calcutta based daily in an interview.

The prime minister's virtual challenge to the Left parties, whose 64 members in Lok Sabha provide a crucial make or break support to the UPA government, comes in the context of strong statements made by CPI-M leader Prakash Karat and CPI A B Bardhan. The Left leaders had warned that the government may have to pay a "heavy price" if it went ahead with the nuclear deal with the US.

The paper quoted the prime minister as saying he was "not angry but anguished" at the harsh tone and tenor Left's reaction and made it clear that the UPA-Left relationship could not be a one-sided affair.

"I don't get angry; I don't want to use harsh words. They are our colleagues and we have to work with them. But they also have to learn to work with us."

The Left parties yesterday downplayed Manmohan Singh's challenge to withdraw their support on the Indo-US civil nuclear deal, maintaining that "we are going to oppose it" and "everything should not be linked with pulling down the government".

While senior CPI(M) leader Sitaram Yechury and CPI's D Raja went roundabout reacting to Singh's statement, their junior allies, RSP and Forward Bloc, were forthright saying the Left support cannot be taken for granted.

Asked whether the Left will withdraw support, Yechury, leader of the CPI(M) in Parliament, said "everything should not be linked with pulling down the government.

Our concern is with the nuclear deal. Governments will come and go but the agreement will remain."

Singh, who had spoken to the Left leaders on Tuesday night after their rejection of the nuclear deal, said there was no immediate response by the Left leaders to what he had told them.

"They haven't thought it through." The Prime Minister felt that the Left had a flawed understanding not just of the 123 agreement but also of India's intrinsic strength and its enhanced status in the world.

Singh said the deal was an honourable one which enlarged India's development options particularly in regard to energy security and environmental protection and doesn't in any way affect our ability to pursue our nuclear weapons programme.