A stand-off between the traditional nuclear nations and a growing number of non-aligned countries, led by Indonesia and Mexico, could jeopardise the survival of the treaty which has helped curb the spread of weapons of mass destruction for 25 years.
A ministerial conference in April is scheduled to vote on the indefinite extension of the NPT. Its backers, including Britain, Russia and the US, admit they are well short of the required majority of the 168 signatory countries. "It is going to be very difficult, there is no doubt about it," said one diplomat after trouble surfaced at a preparatory meeting this week.
At the heart of the NPT, negotiated in 1968, was an explicit bargain that five nuclear powers - the US, Britain, the former Soviet Union, France and China - would work towards abandonment of nuclear weapons in exchange for a commitment from all other countries that they would forswear any military nuclear capability.
While Britain, the US and Russia argue that strides have been made towards denuclearising the world, notably with dismantling superpower stockpiles, many among the Non-aligned Movement are unimpressed.
"We want a commitment that they will come up with a programme of nuclear disarmament," said Miguel Marin Bosch, Mexico's ambassador to the UN in Geneva. "This is like the slavery debate of the 19th century. At the time, it seemed to make sense for those who defended slavery. Now it seems crazy. We think it is crazy to defend having nuclear weapons."
Britain is likely to come under particular pressure for moving ahead with the replacement of Polaris with the Trident system which became operational last month. While there is no suggestion that specific weapons ,should be laid on the table the dissenting countries are expected to set conditions before accepting the treaty's extension. Demands will include a commitment from the nuclear nations to complete negotiations on a comprehensive testing ban, to end all production of fissile materials for military use and provide civil nuclear technology to NPT signatories, and an acceleration in arms reduction.
Complicating the extension talks are regional controversies, especially in the Middle East. Egypt, backed by other Arab states, has served notice it will withhold support for an extension unless Israel, believed to have nuclear capability, gives some assurance of co-operation in NPT activities.
Support for the NPT survives in many capitals, in spite of its many and recognised shortcomings. While several "threshold" countries, believed to have developed nuclear weapons or to be close to doing so, have never signed up, including Israel, India andPakistan, even some signatory countries have come close, at least, to getting through the net, notably Iraq and North Korea.
London, Washington and Moscow remain committed to the indefinite extension. Unless the arguments can be quickly settled, their best hope may be for a compromise, such as an extension of another 25 years.