India said it would remain at the negotiations. But the opposition of one of the three "nuclear threshold" states - India, Pakistan and Israel - could delay adoption of the treaty for which negotiators have been striving for almost 40 years, and, even if it is signed, render it ineffective.
India is is one of eight countries --the five declared nuclear powers and three "threshold" powers - which the official nuclear powers want to ratify the treaty before it comes into force. India also refused to accept that provision yesterday. The official nuclear powers believe that unless the treaty becomes law in the threshold states, which either have nuclear weapons (Israel), or could build them very easily (India and Pakistan), it will be meaningless.
India's Foreign Minister, I K Gujral, said: "The treaty as it has been drafted is a charade. If we want to rid the world of these weapons, then it is the five powers, which have these weapons, which have to do something." India's stand means that the five nuclear-weapons states will have to make some concession to India, including a commitment not to build new nuclear weapons and some sort of timetable for eventual nuclear disarmament, or agree to let the CTBT come into force without India.
The first discussion of an international nuclear test ban treaty began in 1958. Some arms control campaigners fear that if the 28 June deadline is not met, 40 years of work to reach the CTBT will have been in vain. But diplomatic sources last night said a further delay for negotiations would not kill the treaty.
India signed the Partial Test Ban Treaty (PTBT) in 1963, which prohibited tests in the atmosphere. It conducted a so-called "peaceful nuclear explosion" under the Rajasthan desert in 1974 but has not exploded a nuclear device since, although the US State Department warned of indications that it was preparing to conduct a test there earlier this year.
India is, therefore, a "threshold" nuclear state, which could assemble a workable nuclear device quickly. Pakistan, with which India has fought three wars since 1947, has a proven missile-warhead design and could also assemble weapons quickly. Current Indian doctrine envisages keeping components of nuclear weapons separate, which can be assembled when needed to carry out a "second strike" in response to attack by Pakistan or China.
When the CTBT was first proposed, a ban on testing would have acted as an effective constraint on the development of new weapons by the established nuclear powers and on nuclear proliferation.
To ensure new nuclear weapons worked it was necessary to test them, and nuclear stockpiles also had to be tested periodically to check they still worked. However, modern computer simulation techniques have made tests unnecessary. France's nuclear tests in the Pacific last year were the last, and Monday's agreement between France and the US to share nuclear data has further obviated the need for tests.
Some experts also question whether India would need to test nuclear weapons. Pravin Sawhney, a former Indian Army officer and a research fellow at the Royal United Services Institute in London, said India could build "boosted fission" devices --nuclear fission bombs with a fusion component similar to the first British "H-bombs", and with a yield of up to 500 kilotons - without tests.
"The Indian government has invested heavily in super-computing and related software", Mr Sawhney wrote in the Institute's journal.
"It is assumed Israel is capable of manufacturing nuclear weapons and has deployed them. India's nuclear infrastructure and scientific base is as well developed as that of Israel. If the latter can make nuclear weapons without testing, India can certainly do it".