WASHINGTON (Reuters) -- Aiming to cement closer ties with India, President Bush is to sign legislation on Monday that is a major step toward allowing New Delhi to buy U.S. nuclear reactors and fuel for the first time in 30 years.

The bill was approved overwhelmingly by Congress on December 9. But three other approvals -- by the 45-nation Nuclear Suppliers Group, the International Atomic Energy Agency and the U.S. Congress -- are still needed before American nuclear transfers to India can take place.

Still, some analysts say winning passage of the law Bush is due to sign at the White House was the hardest hurdle.

Critics decried the bill -- which makes changes in the U.S. Atomic Energy Act -- as a historic mistake that undermines efforts to curb the spread of nuclear weapons. But the Bush administration and its allies insisted nuclear commerce to expand electricity generation in India will foster a broad range of ties with the rising South Asian power and open up billions of dollars in trade for U.S. companies.