By Sanjoy Majumder
BBC News, Delhi
India's nuclear tests shocked the world
A retired atomic scientist who was closely associated with India's 1998 nuclear tests has said they were not as successful as was claimed.
K Santhanam said one of the tests - on a hydrogen bomb - had not worked, and that India would have to carry out more tests for a credible nuclear deterrent.
His statement has been dismissed by the government and his former colleagues.
The Indian tests led to similar tests by Pakistan, raising fears of a nuclear conflict between the two countries.
K Santhanam is a respected Indian atomic scientist who was project director of the 1998 nuclear tests.
He now says that one of the five tests that were carried out, in which a thermonuclear device or hydrogen bomb was detonated, did not perform as well as expected.
He also said that everyone associated with the tests immediately recognised that something had gone wrong.
If his statement is accurate it points to a massive cover-up by India and also confirms what many in the West suspected at the time - that the nuclear devices India tested were not as powerful as had been thought.
India's government has dismissed Mr Santhanam's claim, which has also been disputed by senior officials of the BJP-led government which carried out the tests.
The scientist says that India is coming under pressure to sign up to the Comprehensive Nuclear Test Ban Treaty, a move he says would be disastrous since he contends that the country does not yet have a credible nuclear deterrent.