That move was seen as a dramatic policy shift as Washington had previously frowned on India's status as an unofficial nuclear power.

The International Thermonuclear Experimental Reactor (ITER), is a project run at the moment by five partners apart from the EU -- China, Japan, South Korea, Russia and the United States -- and all have to agree to let India into the club.

"By bringing in India, more than half on the world's population is represented at ITER," Antonia Mochan, the European Commission's spokeswoman on science and research, said.

"It's important to have such a scientific experiment which could have such huge ramifications for energy, and its important to do that with people who could bring so much scientific know-how."

European cooperation with New Delhi on the project was a separate issue from India's avoidance of the nuclear Non-proliferation Treaty, she said.

"There is the non-proliferation issue and we are pursuing that with the Indians as part of our external relations policy ... This is nuclear fusion to be used as energy -- this doesn't have any military potential," Mochan said.

The aim of the reactor is to mimic the way the sun produces energy by heating hydrogen atoms to 100 million Celsius -- much hotter than centre of the sun -- to achieve a fusion which would produce helium and yield huge amounts of energy.

Scientists say 1 kg of fusion fuel would produce the same amount of energy as 10 million kg of fossil fuels.

They know the theory works because the hydrogen bomb works, but they face the problem of trying to fuse the atoms in a controlled manner, not least to sustain temperatures of 100 million Celsius for long enough to generate power.

Critics of the project, which began life in 1985 as a showpiece of Cold War diplomacy between the United States and the Soviet Union, say it is too expensive, unrealistic, with too long a wait for results.