As the talks with Iran got underway in Baghdad this morning, the BBC put out an eyebrow-raising story that ministers in the National Security Council held a discussion last week on what Britain should do in the event of an Israeli strike on Iran aimed at damaging its nuclear programe, and the legality of any such UK action.

One of the surprising aspects of the story is why the NSC would only be having this discussion now, while the threat of some kind of Israeli strike has hung heavy in the air for many months. The Guardian´s Nick Hopkins wrote about UK military contingency planning last November.

Any UK action would be extremely divisive for the coalition so perhaps it is not surprising that the issue gets a frequent airing at the NSC. Andrew Rawnsley argued in The Observer in March that Iran was the issue most likely to tear the government apart. The prospect clearly worries Liberal Democrat members of the coalition who are adamant that the UK should operate strictly within the boundaries of international law, and not repeat the mistakes of Iraq.

In a speech to the Liberal Democrat Lawyers´ Association at the end of 2010, the international lawyer, Philippe Sands, laid out the legal scenery in the wake of Iraq and the Chilcot Inquiry and explored how it was applicable to Iran. The only possible justifications for an attack, he said, fell under Chapter VII of the UN Charter authorising force "to maintain or restore international peace and security". But Sands argues it is generally accepted that the judgement on when Chapter VII applies is the job of the Security Council, not individual governments.

That leaves the right to act in self defence, under Article 51 of the UN Charter, and the question of whether and when "pre-emptive" or "anticipatory" self-defence can be justified, particularly in cases involving suspected WMD development. After the experience of the ´Bush Doctrine´ in Iraq, such a justification for any attack would have to cross a high threshold of proof.

The key issue here is one of "imminence" of the perceived Iranian threat, as my colleague, Chris McGreal, discusses in his report last month on the legal debate in the US. The current judgement of the US intelligence community, expressed in the current National Intelligence Estimate (NIE) that Iran has not made any decision to build a weapon, is an important, arguably decisive, exhibit in this debate. Sands told me this morning:

On the basis of what is in the public domain today, it is not easy to see how the UK could support the use of force as a matter of international law.

It seems highly unlikely under present circumstances that the UK would be involved in any pre-emptive strike on Iran. A more realistic scenario is the possibility that an Israeli strike triggers Iranian reprisals in the form of attacks on shipping or oil facilities in the Gulf, with British armed forces being called in to help keep the Gulf open for business. The legal standing of that action would depend on circumstances but would be sturdier than any involvement in a first strike.

While the talks in Baghdad - and with the IAEA in Vienna and Tehran - continue, the chances of even an Israeli strike are very small. The story may have been leaked on the morning of the Baghdad talks in order to remind the Iranians that the alternatives to negotiating could be severe. However, if those talks break down, or if none of the fundamental issues are resolved by next year, after the US elections, then all the contingency planning underway now will become sharply relevant again. (guardian.co.uk © Guardian News and Media Limited 2012 05/24/12)