March 22 (Reuters) - Gordon Brown may lead efforts to revive the Nuclear Non-Proliferation Treaty (NPT) as a way of tackling Iran's atomic ambitions, his most pressing foreign policy challenge once he becomes British prime minister.

Sources close to Brown say his aides are discussing ways of reinforcing the NPT, a pact which has neither prevented several states acquiring nuclear arms nor persuaded others to disarm.

Here are some key facts about the treaty, regarded as the cornerstone of global efforts to prevent the proliferation of nuclear weapons:

* WHAT IS THE NPT?

-- The objective of NPT, ratified in 1970, is to halt the spread of nuclear weapons while promoting development of peaceful uses of nuclear energy.

-- The treaty defines nuclear-armed states as those that "manufactured and exploded a nuclear weapon or other nuclear device prior to January 1, 1967." Those countries are Britain, China, France, Russia and the United States -- the permanent members of the United Nations Security Council.

* SIGNATORIES:

-- A total of 188 countries are now part of the treaty. Nuclear states are bound not to transfer nuclear weapons or to help non-nuclear states to obtain them.

-- Non-nuclear signatories must neither develop nor acquire such weapons but are given an assurance of assistance to develop nuclear energy for peaceful purposes, monitored by the Vienna-based International Atomic Energy Agency (IAEA).

-- However, several non-signatories to the treaty are, or are believed to be, nuclear weapons states, including India, Pakistan and Israel. South Africa signed the treaty in 1991 and admitted producing nuclear devices until 1970.

-- North Korea signed in 1985, but withdrew in 2003 after a dispute with the IAEA, whose weapons inspectors were monitoring a nuclear reactor suspected of producing plutonium for nuclear weapons.

-- At a conference on the treaty's continuation in May 1995, the NPT was extended indefinitely.

-- The treaty is divided into 11 articles, including one that enables a state to withdraw "if it decides that extraordinary events... have jeopardised the supreme interests of its country." A state must give three months notice to other treaty parties and the United Nations Security Council.

-- Article VI commits the signatories to pursuing negotiations on a treaty for "general and complete disarmament," but little progress has been made so far.

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