Time is now running out if the CTBT is to be ready for signing at the UN General Assembly in New York in September, as had been scheduled. At the moment India is posing the biggest problem, by insisting that the CTBT must promote disarmament as well as curtailing tests. India wants the treaty to include a pledge by the nuclear powers to dismantle their existing nuclear weapons. If India refuses to sign, as is widely expected, the treaty will not be legally binding.
Meanwhile China, backed by India, is objecting to proposed measures to monitor and verify the global test ban, saying that such on-site inspections may be used by the West for intelligence-gathering.
The United States and Russia believe Peking will agree to a compromise on this sticking point, and the Chinese foreign minister, Qian Qichen, last week said he hoped the treaty would be signed in September.
In June China dropped its initial insistence that "peaceful" nuclear explosions ought to be made be exempt from the test ban, although it still wants the issue reassessed after 10 years.
China's underground nuclear explosion yesterday morning, at its Lop Nor site in western Xinjiang province, was its 45th since Peking started tests in 1964. The test was very small - estimated to be the equivalent of about 5,000 tons of TNT, a quarter to a third of the size of the Hiroshima bomb and minimal by modern standards. The small size of the test suggests it may have been conducted to calibrate instruments for subsequent computer simulation of nuclear explosions, or to test a particular component of a nuclear device, such as a trigger mechanism.
By halting its programme now, China has ensured that it will sit level with Britain in the league table of nuclear tests.
Peking's reply to worldwide criticism for being the last country to observe the moratorium has always been that it has conducted fewer tests than the other big nuclear powers.
According to Greenpeace, the final nuclear test scorecard reads: the United States, 1,030; the former Soviet Union, 715; France, 210; the UK, 45; China, 45; and India, probably 1.
The Australian Foreign Minister, Alexander Downer, said: "The ending of Chinese nuclear testing means that no nuclear weapons state is any longer conducting nuclear tests. This test we had today we hope will be the last nuclear test that will ever take place."
China's decision to hold its last test before the Geneva talks resumed suggests that Peking was anxious to minimise a potentially hostile global reaction. Peking had already announced in June that it would be conducting one more test, which analysts warned could be disruptive if it took place during the CTBT negotiations.
Yesterday, after years of being on the defensive about its continuing tests, Peking was seeking the moral high ground.
"Such an important decision by China is not only a response to the appeal of the vast number of non-nuclear weapon states, but also a concrete action to promote nuclear disarmament," a government statement said.
China is believed to have the smallest nuclear arsenal of the five nuclear powers. The government statement called on nuclear powers "drastically to reduce" nuclear stockpiles.
China's Asian neighbours, however, yesterday criticised Peking for conducting one last test, although they welcomed the new moratorium.
Anti-nuclear groups, independent analysts and diplomats in the West saw the test as a boost to their hopes for a comprehensive and permanent nuclear test ban.
"The main thing is the Chinese have now announced a moratorium", the Foreign Office said last night. "We share the international community's wish that yesterday's test should be the last ever."
It said it hoped the moratorium would encourage all states to conclude the CTBT on the basis of the text proposed by Jaap Rameker, the Dutch ambassador to the Conference on Disarmament in Geneva.
According to Suzanne van Moyland, of the London-based Verification Technology Control Institute (Vertic), the test was a "boost" to hopes that it would be the last and also an exercise of "leverage" by China in the final stages of the CTBT negotiations, which stalled in June.
Rebecca Johnson, of the Disarmament Intelligence Review, said: "China's timing is clearly aimed at giving a boost to early conclusion of the Treaty, as China realised that it would be very difficult to test once the Treaty was agreed".