K Santhanam, who was director for 1998 test site preparations, told the Times of India in an interview that the yield of thermonuclear explosions was actually much below expectations and the tests were perhaps more a fizzle rather than a big bang.
In nuclear parlance, a test is described as a fizzle when it fails to meet the desired yield.
He emphasized the need for India to conduct more tests to improve its nuclear weapon programme.
The test was said to have yielded 45 kilotons (KT) but was challenged by western experts who said it was not more than 20 KT.
The exact yield of the thermonuclear explosion is important as during the heated debate on the India-US nuclear deal, it was strenuously argued by the government's top scientists that no more tests were required for the weapons programme. It was said the disincentives the nuclear deal imposed on testing would not really matter as further tests were not required.
According to security expert Bharat Karnad, Santhanam's admission is remarkable because this is the first time a nuclear scientist and one closely associated with the 1998 tests has disavowed the government line.
"This means the government has to do something. Either you don't have a thermonuclear deterrent or prove that you have it, if you claim to have it," said Karnad.
The yield of the thermonuclear device test in 1998 has led to much debate and while western experts have stated that it was not as claimed, BARC has maintained that it stands by its assessment.
Indian scientists had claimed after the test that the thermonuclear device gave a total yield of 45 KT, 15 KT from the fission trigger and 30 KT from the fusion process and that the theoretical yield of the device (200 KT) was reduced to 45 KT in order to minimise seismic damage to villages near the test range.
Challenging the claims, British experts declared the combined yield for the fission device and thermonuclear bomb not more than 20 KT.