The ambassador to the United Nations Disarmament Conference in Geneva, Sir Michael Weston, will make a formal declaration today outlining British undertakings.
China gave similar assurances yesterday, while the other three recognised nuclear states - the United States, France and Russia - are expected shortly to follow suit.
The British statement says the UK would not use nuclear weapons against any non-nuclear state that has signed the treaty, unless that state joined in alliance with a nuclear power to attack this country, its allies, or a state to which Britain has a security commitment. Britain would seek immediate UN security council action were any signatory state without nuclear arms to face aggression or threatened attack by a nuclear power.
The co-ordinated pledges by the five nuclear states are intended to support a diplomatic campaign to win a convincing majority among the numbers 174 signatories to the non-proliferation treaty (NPT) for its indefinite extension. The treaty, 25 years old, comes up for renewal this spring.
"The Treaty is the cornerstone of the only world-wide nuclear non proliferation regime we've got," a British official said yesterday. "Anything less than an indefinite extension would in effect be a vote of no confidence."
Non-aligned nations have expressed doubts about any such extension, while the debate has become entangled by nuclear rivalries within the Middle East. Some countries would prefer to see the NPT extended only for a fixed period - five or 25 years - while others say they will not renew it unless Israel agrees to sign. These arguments have bedevilled the effort to achieve a "moral" majority well in excess of the necessary 88 votes.
British officials said the campaign will start in earnest when the Foreign Secretary, Douglas Hurd, flies to New York to address the opening session of the NPT conference, which begins on 17 April.
But the official British position has been challenged by critics at home, especially the Campaign for Nuclear Disarmament (CND) which maintains that the developing countries are right to accuse the nuclear powers of hypocrisy.
The point about yesterday's security guarantees is that they serve a double purpose. They are meant to reassure nations without nuclear weapons that the five weapons states intend to create a stable framework for future arms reductions.
But by restricting the guarantees to signatories of the NPT they also contain an implicit warning to nations that stay outside it.