President Bush has signed into law a bill that reverses three decades of U.S. policy and allows American businesses to enter India's multibillion-dollar nuclear market.

"It's a big deal," the president said Wednesday at a ceremony in the East Room. He said the measure would build on the growing ties between the world's two largest democracies, the United States and India.

The signing of legislation approving U.S.-Indian civilian nuclear cooperation is the result of three years of work by Indian officials and the Bush administration, which says the deal will let the U.S. solidify ties with a democratic and responsible rising power. Opponents say it could spur a nuclear arms race in South Asia.

The State Department said that U.S. Secretary of State Condoleezza Rice and Indian External Affairs Minister Pranab Mukherjee would sign the overall bilateral nuclear cooperation accord Friday. It will eventually allow American businesses to begin selling nuclear fuel, technology and reactors to India in exchange for safeguards and U.N. inspections at India's civilian, but not military, nuclear plants.

U.S. lawmakers last week gave final congressional approval to legislation authorizing civilian nuclear trade with India, which built its atomic bombs outside the Nuclear Nonproliferation Treaty. India has faced a nuclear trade ban since its first atomic test in 1974.

Now that Bush has signed the authorizing legislation, he is required to certify that the agreement with India is consistent with U.S. obligations under the Nuclear Nonproliferation Treaty, designed to limit the spread of nuclear weapons. He must also certify that it is U.S. policy to cooperate with international efforts to further restrict transfers of technology related to uranium enrichment and the reprocessing of spent nuclear fuel.

Critics in India argue the constraints compromise their country's right to conduct nuclear bomb tests.