President George W Bush has signed into law a historic agreement allowing the United States to export civilian nuclear fuel to India.
Mr Bush and Mr Singh finalised the deal in India in March
The deal was finally approved by Congress earlier this month.
Mr Bush and Indian Prime Minister, Manmohan Singh agreed on the deal in principle in July, 2005.
Critics say it will harm efforts to stop the spread of nuclear weapons as India has not signed the nuclear Non-Proliferation Treaty (NPT).
Even with Mr Bush signing the legislation, it still has three more hurdles to overcome.
President Bush described the new law as an important achievement for the whole world.
"The bill will help keep America safe by paving the way for India to join the global effort to stop the spread of nuclear weapons," he said.
"After 30 years outside the system, India will now operate its civilian nuclear energy programme under internationally-accepted guidelines and the world is going to be safer as a result."
Mr Bush has insisted that the deal "will strengthen the strategic relationship between America and India and deliver valuable benefits to both nations".
Under the deal, energy-hungry India will get access to US civil nuclear technology and fuel, in return for opening its civilian nuclear facilities to inspection.
But its nuclear weapons sites will remain off-limits.
Indian opposition leader LK Advani denounced the deal in a debate in the Indian parliament on Monday, saying it would make India "a client state of the United States".
"The primary objective is to cap, roll back and ultimately eliminate [India's] nuclear weapons capability," he told legislators.
The government's communist allies are also opposed to the agreement.
The deal does not have to be ratified by the Indian parliament. However, the opposition could try to force a full debate followed by a vote to reject the agreement.
There are three more stages before the agreement actually starts working.
- India and the US have to agree terms for the lucrative trade deal by which the US sells India nuclear technology and fuel - the US Congress has to ratify the deal
- The International Atomic Energy Agency has to approve a separate nuclear inspection programme
- The Nuclear Suppliers Group, an assembly of nations that exports nuclear material, has to give its approval.
Once on opposite sides of the Cold War fence, India and the US have become allies with close economic, political and even defence ties.
NUCLEAR POWER IN INDIA
India has 14 reactors in commercial operation and nine under construction
Nuclear power supplies about 3% of India's electricity
By 2050, nuclear power is expected to provide 25% of the country's electricity
India has limited coal and uranium reserves
Its huge thorium reserves - about 25% of the world's total - are expected to fuel its nuclear power programme long-term
Source: Uranium Information Center
Correspondents say that India sees the deal as a tacit acceptance of its emergence as a global nuclear power.
But some say that by making an exception for India, the US will find it difficult to rein in the nuclear ambitions of North Korea and Iran.
The proposed agreement reverses US policy to restrict nuclear co-operation with Delhi because of its refusal to sign the nuclear Non-Proliferation Treaty (NPT) and its testing of nuclear weapons in 1974 and 1998.
Mr Bush and Mr Singh finalised the agreement in India in March.
Some critics of the deal say it could boost India's nuclear arsenal. They say it sends the wrong message to countries like Iran, whose nuclear ambitions Washington opposes.
India has made clear that the final agreement must not bind it to supporting the US policy on Iran and does not prevent it from developing its own fissile material.