By Paul Reynolds World Affairs Correspondent, BBC News website

As diplomatic attempts continue in the UN Security Council to get Iran to suspend its nuclear enrichment activities, the question has been raised about an American attack on Iran and whether it would be legal under international law.

Iran says its nuclear programme is

If the US decided to attack Iran, it would probably claim that it was acting pre-emptively and exercising an inherent right of self-defence under the UN Charter.

One can rule out the US taking the other main legal path by which one state can attack another - an authorisation of force by the Security Council. Russia and China, both veto holders, are opposed to sanctions against Iran, let alone military action.

And nor would it invoke the growing doctrine of a humanitarian intervention, as the conditions needed for that do not apply.

So the US would probably seek to justify an attack under the self-defence principle, and it would first of all have to outline the nature of the threat.

Currently, this would refer to Iran's previously secret development of enrichment technology, and therefore its forfeiture of trust; its refusal to follow Security Council demands to suspend enrichment; and its president's hostile comments on Israel's right to exist.

All of these would be declared a threat to the US, its interests and to regional and world security. At some future date, the US might bring forward further arguments, depending on how Iran's nuclear programme develops.

Article 51

Having defined the threat, the US would then invoke Article 51 of the UN Charter, which allows self-defence.

This article says: "Nothing in the present charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security."

To get round the phrase "if an armed attack occurs", the US would say that international law does not require that an attack is actually taking place, and that its own new doctrine of pre-emption, an extension of the self-defence principle, was being implemented.

It justified pre-emption in a National Security Strategy document in 2002, after the attacks of 11 September 2001:

"The greater the threat, the greater is the risk of inaction - and the more compelling the case for taking anticipatory action to defend ourselves, even if uncertainty remains as to the time and place of the enemy's attack. To forestall or prevent such hostile attacks by our adversaries, the United States will, if necessary, act pre-emptively."

The US might say that it was acting in protection of or at the request of Israel, which could argue that it was under a greater threat than the US itself.

Collective defence is allowed by the UN if the original state claiming self-defence asks for help.

It is possible that, if the Security Council ever agreed a resolution under the enforcement of Chapter Seven of the UN Charter, which orders a member state to comply, the US could declare that it was enforcing it unilaterally.

International law

Would such arguments be accepted in international law?

There is currently no basis for an American attack on Iran under Article 51 Elizabeth Wilmshurst

There is some legal backing for the principle of not waiting too long.

A British judge, Dame Rosalyn Higgins, who was made president of the International Court of Justice in February, said before she joined the court: "In a nuclear age, common sense cannot require one to interpret an ambiguous provision in a text in a way that requires a state passively to accept its fate before it can defend itself."

However, the general view among international lawyers is that there has to be the threat of an "imminent" attack.

British Attorney General Lord Goldsmith, who used a complex series of Security Council resolutions on Iraq to justify the 2003 invasion, was critical of pre-emption in the House of Lords in April 2004: "International law permits the use of force in self-defence against an imminent attack, but does not authorise the use of force to mount a pre-emptive strike against a threat that is more remote."

There is therefore a fairly fundamental divergence between the US doctrine and the view of much of the rest of the UN membership. At the very least, there is no settled opinion.

The question of imminence

Elizabeth Wilmshurst, senior fellow in international law at the British think tank Chatham House, who resigned as a legal adviser to the Foreign Office because she felt the invasion of Iraq was illegal, told the BBC News website: "There is currently no basis for an American attack on Iran under Article 51. There certainly is not a case for self-defence at the moment.

"You do not have to wait for an attack but the threat has to be real and imminent."

She did not think the conditions for a self-defence argument existed. "Does enrichment of uranium count as a threat?" she asked. "It has not been weaponised. Is there a threat?"

Nor did she accept that the US could enforce a Chapter Seven resolution by itself. "This requires a further resolution authorising force and is a settled view," she said.

That an attack is illegal is also a view shared by former British Foreign Secretary Jack Straw. He told reporters the other day that an Article 51 action could not be justified.

The new Foreign Secretary, Margaret Beckett, has not gone that far, saying only that nobody had any "intention" of attacking Iran.

British Prime Minister Tony Blair has pointedly refused to say that an attack is "inconceivable", a word used by Mr Straw, but whether this is a tactical use of language to rattle Iran or whether it foretells potential British support for an attack is not clear.

Ms Wilmshurst accepted that Israel might regard itself as threatened, given the remarks made by President Mahmoud Ahmadinejad.

But she added: "Israel would have to take an objective, realistic view as to whether there was a real threat, and I am doubtful at the moment."

The Caroline incident

Much of the traditional doctrine on self-defence comes from an incident in 1837 near the Niagara Falls, in which a boat called the Caroline was attacked and tipped over the Falls by British forces that moved into American waters from Canada. The boat was being used by Canadian rebels preparing an attack.

Some very elegant diplomatic exchanges between US Secretary of State Daniel Webster and British Foreign Secretary Lord Ashburton led to the acceptance of Webster's principles of pre-emptive self-defence. These held that it was justified only in cases in which the "necessity of that self-defence is instant, overwhelming, and leaving no choice of means, and no moment for deliberation".

The UN Charter basically adopted that rule, and a highlevel group which looked at UN reform in 2004 said that "Article 51 needs neither extension nor restriction in its long understood scope".

The General Assembly confirmed that view. However there remains some debate about how "imminent" a threat has to be, and how large.

The doctrine of pre-emption has therefore not received widespread international backing. Last year, Chatham House sent a questionnaire about self-defence to 13 international lawyers in Britain. As a result, a number of principles were drawn up to give precision to Webster's phrasing.

These stressed the importance of imminence.

Post-9/11 style pre-emption was not endorsed.