It will be seen as an effort by London and Washington to help France to rebuild diplomatic and political bridges in the region, shattered by the resumption of tests last month. Paris will also commit itself to closing its test facilities at Mururoa once it has completed its experiments in May.
All three Western nuclear powers - France, Britain and the US - are expected to pledge adherence to the 1985 Treaty of Rarotonga, which established a nuclear-free zone in the South Pacific.
The announcement, to be made simultaneously in Paris, London and Washington, is likely as early as tomorrow, diplomatic sources in New York confirmed. "It is 99 per cent certain, although there are a couple of wrinkles left to iron out," one European diplomat said.
Suzanna van Moyland of the Vertic nuclear non-proliferation pressure group, said: "This is a very positive development for the region. But there is no reason why Britain and the US should not have signed long ago. It is interesting that they are holding back for France."
The timing of the initiative is far from arbitrary. On Saturday, leaders of 150 nations arrive in New York for three days of speeches to celebrate the 50th anniversary of the United Nations. Many feared the event would be marred by protests against the French President, Jacques Chirac.
Britain has been searching for a way to mitigate criticism of its failure to join the condemnation of France. John Major will be asked to explain Britain's low profile at the summit of Commonwealth heads of state and government next month in New Zealand.
By signing protocols attached to the Treaty of Rarotonga, the three countries will commit themselves to its main provisions forbidding the use, storage, testing or dumping of any nuclear explosive devices in the South Pacific. Russia and China are already signatories. Until now Britain in particular has been hesitant about such a pledge.
Reaction to the announcement among nations that have been most upset by the French tests is not likely to be ecstatic. While announcing its intention to join the treaty, France can still give no indication of when exactly it will put pen to paper. Only at that time will it be obliged to cease using Mururoa for nuclear testing. None the less, until this point France has never given any undertaking to close its Pacific nuclear facilities. "The fact that France and the other Western powers are going to take this amazingly important step should help cool tempers," a European diplomat insisted.
Ironically, Australia only yesterday voiced formal disappointment before the UN's General Assembly that the three countries had still not joined the Rarotonga Treaty. The deputy Australian ambassador to the UN, Richard Rowe, said an announcement reversing that stance would mitigate some, but not all, of the ill-feeling against France. "It's good news as far as it goes, but our position still remains that France has got to stop testing."
On the broader issue of testing, the US is pressing for a statement from the nuclear powers setting 30 April 1996 as the deadline for agreeing a final text in the Comprehensive Nuclear Test Ban talks in Geneva. President Bill Clinton, who may commit his administration to the deadline when he addresses the UN on Sunday, is anxious to accelerate work towards the test ban.
If a text can be settled by the end of April, the way would be clear for final signature in October next year, enabling him to claim credit ahead of the US presidential elections in November. There is scepticism among European officials whether an April deadline is practicable, however. One said that there was a reluctance to appear to be "dragged along by the Americans".