From an excerpt of  R Chidambaram’s memoirs:

Money was allocated from a ‘secret fund’ maintained by the Defence Research and Development Organisation (DRDO). In the 1990s, Prime Minister PV Narasimha Rao told scientists ‘to be prepared to carry out the tests anytime on 10 days’ notice’. But the tests were put off for fear of the economic fallout. Atal Behari Vajpayee gave the go-ahead soon after he became the PM in 1996. Scientists were in Pokhran and had placed the main nuclear device in the chamber and sealed it. They were ready to place the same in the shaft when the test was called off due to the no-confidence motion against the Vajpayee government. The tests were finally conducted in 1998. “The extra time of two years allowed us to develop more sophisticated devices,” Chidambaram says in the book.