India's parliament began debate on a vote of confidence in Prime Minister Manmohan Singh's government on Monday that will decide the fate of a nuclear deal with the United States and could trigger a snap election.

The vote, due on Tuesday, is so close that several MPs who are ill may be flown or wheeled in from hospital, and others, in jail for crimes such as murder and extortion, have been granted temporary release.

If the Congress party-led government falls there will almost certainly be an election this year. It would also likely lead to the scrapping of the civilian nuclear agreement and throw economic policy into limbo just as inflation rises dangerously.

Arriving at parliament on Monday morning, Prime Minister Singh gave a "V" for victory sign.

"We will prove our majority on the floor of the house," he said, providing a boost to India's stock market, which is watching the debate nervously. At 11:57 a.m. (7:27 a.m. British time), the main 30-share BSE index was trading up around 1.3 percent.

"I welcome this opportunity of this house to review our record," he said, after moving the confidence motion. "I have no doubt that the people of India, when they consider what we have done, will reaffirm their confidence in us."

Numbers are in flux, but newspapers said the decision may come down to one or two votes in the 543-member house.

"Vote looks neck and neck" was the headline of The Asian Age.

The nuclear deal would grant India access to foreign nuclear fuel and technology, unlocking billions of dollars in investment.

But the government's communist allies withdrew their support in protest, saying the deal made India a pawn of Washington.

The vote essentially pits the Congress-led coalition in favour of the deal against the communists and a coalition led by the Hindu-nationalist Bharatiya Janata Party (BJP). The BJP says the nuclear deal limits India's ability to test nuclear weapons.

A host of smaller regional and caste-based parties hold the balance. It is unclear which way they will vote amid a flurry of horse-trading that even included the re-naming of an airport to honour the father of one wavering member of parliament.

A government defeat would be a boost for the BJP, which has won a string of state elections this year against a backdrop of rising inflation and criticism that millions of poor Indians were not benefiting from the booming economy.

BJP leader L.K. Advani told parliament the government was likely to fall.

"The UPA government today is like a patient in the ICU," he said, after protests from government benches. "Naturally the first question is: Is it going to survive or not?"

Advani said his party did not oppose a strategic relationship with the United States, but said the nuclear deal made India "a subservient partner".

The parliamentary debate will be followed by an electronic vote, expected on Tuesday evening.

SHADY DEALS

Weak party discipline, under-the-table deals and accidents such as MPs falling ill make the result impossible to predict.

"The problem is that money changes hands, dirty deals are struck and MPs do a volte-face nonchalantly," said political commentator Amulya Ganguli. "It's hard to predict when the race is so tight."

A week ago the government was confident of securing a majority with the support of the regional Samajwadi Party (SP), which replaced the communists as its parliamentary support.

Since then there have been signs of a rebellion in the ranks of the SP, including the defection to an opposition group of senior party member Shahid Siddiqui.

Two small political groups - the JD(S) and RLD with six MPs in all - also decided to vote against the government.

Other MPs have vacillated over voting for a government already battling mounting unpopularity ahead of a general election due by next May.

Analysts say the government is hoping for some abstentions in the opposition camp.