Communist partners of India's ruling United Progressive Alliance Tuesday withdrew their support for the governing coalition after failing to resolve differences over an India-US civilian nuclear deal.

The left-wing parties said they took the decision soon after Prime Minister Manmohan Singh's statement that the government would approach the International Atomic Energy Agency for India-specific safeguards to implement the deal.

Prakash Karat, general secretary of the Communist Party of India (Marxist), which leads the four leftist parties, told reporters they would formally withdraw support on Wednesday, a day ahead of Singh's return from the G8 summit in Japan.

'In view of the prime minister's announcement, that time has come (to withdraw our support),' Karat said.

He read a letter sent to External Affairs Minister Pranab Mukherjee informing that the parties which have provided the Congress-party led UPA with a parliamentary majority for over four years were quitting the coalition.

'We have asked the president (Pratibha Patil) for an appointment tomorrow morning so we can go and formally withdraw support,' Karat added.

On Monday, Singh told reporters on his plane to Japan that New Delhi would approach the IAEA to submit the safeguards text for board approval 'very soon.'

Top communist leaders said they would urge Patil to ask the government to prove its majority on the floor of the parliament.

The main opposition Bharatiya Janata party also demanded that the government seek a vote of confidence in the parliament.

The communists oppose the nuclear deal, saying it would compromise on India's strategic sovereignty and make the country a stooge of 'US imperialism.'

The leftist bloc also includes the Communist Party of India, the Revolutionary Socialist Party and the Forward Bloc, with a combined total of 59 members in the 543-member Indian lower house of parliament.

But the leading Congress party claimed it had the numbers to sustain a parliamentary majority for the ruling coalition in case a no-confidence motion were called.

The UPA government, whose five-year term ends in May 2009, has ruled out new elections.

'We have the numbers and we will prove our majority in the Lok Sabha (lower house),' Congress spokesman Shakeel Ahmed told the IANS news agency.

Soon after the withdrawal by the leftists, the Samajwadi Party (SP), once Congress's arch-foe, pledged its support to the embattled UPA coalition.

'We welcome the nuclear deal and support Prime Minister Manmohan Singh's decision to go ahead to the IAEA for the next step in the deal,' SP president Mulayam Singh Yadav said.

With the withdrawal of leftist support, the ruling coalition will be reduced to 226 members in the parliament, 46 seats short of the majority figure of 272.

But with backing from SP, the UPA could gain the support of 265 members and securing seven votes for the floor test would not be problematic as there were enough independents to convince, political analysts said.

Under the civilian nuclear agreement, the US has promised to provide India with access to civilian nuclear technology and nuclear fuel in exchange for international safeguards on India's civilian nuclear reactors.

After reaching a safeguards agreement with the UN, New Delhi has to lobby for an India-specific waiver with the 45-nation Nuclear Suppliers Group and later send the agreement for final ratification by the US Congress.

India's political crisis comes as Delhi is under pressure from Washington to finalize the deal. Analysts say time is running out for the deal, with new US elections slated for November.

The government says the accord is historic and would end the country's three-decades of nuclear isolation. It asserts that the deal is vital towards meeting India's soaring energy needs for maintaining its high economic growth.

According to Indian officials, Singh is due to meet US President George W Bush in Japan on Wednesday. They are scheduled to hold discussions on the next steps in the nuclear deal which both leaders had announced in 2005.