What is the legality of the US and Israeli attacks on Iran?

Were the initial strikes by the US and Israel lawful?

There was consensus among legal experts the Guardian spoke to that the initial strikes were unlawful.

The Israeli president, Isaac Herzog, told BBC Radio 4’s Today programme the fact that Iran was “planning a bomb” was enough to justify the attacks. Under article 51 of the UN charter there is a right to self-defence in response to an armed attack. A broader interpretation of international law has been that a state has a right to use force in response to an “imminent threat”.

Susan Breau, a professor of international law and a senior associate research fellow at the Institute of Advanced Legal Studies, said: “Even the doctrine of imminent [threat of] use of force is very controversial. Academics are divided on what it actually means. But in this case, there doesn’t seem to be any evidence of an imminent threat by Iran.”

Several experts cited Donald Trump’s claim to have obliterated Iran’s nuclear programme last year as evidence that directly countered the suggestion of an imminent threat.

Herzog highlighted Iran’s threats to “annihilate” Israel, but Victor Kattan, an assistant professor of public international law at the University of Nottingham, said: “Having blood-curdling rhetoric or threatening violence in and of itself does not give a state the right to use pre-emptive force.”

Is the UK’s position of conducting a defensive operation lawful?

Legal experts were once more unanimous but this time in agreeing that the answer to this question is unclear. The UK took the unusual step of releasing a summary of its legal position that it is acting “in the collective self-defence of regional allies who have requested support”. Under article 51, there must be a request from the victim for intervention.

The UK’s statement said: “The UK has military assets flying in the region to intercept drones or missiles targeting countries not previously involved in the conflict. In addition, the UK has responded to a US request which will facilitate specific and limited defensive action against missile facilities in Iran which were involved in launching strikes at regional allies.”

It was the US request that caused most concern among the lawyers, particularly if it would amount to protecting US bases in the region.

Philippe Sands, a professor of law at University College London, said: “If the United Kingdom base at Cyprus has been attacked, of course the United Kingdom can defend itself against such attacks, provided the response is proportionate. The argument that the United Kingdom is entitled to provide defensive support in relation to a use of force which is manifestly unlawful – as the attack on Iran by the US and Israel was – is, on its face, far more problematic, as is the distinction between a defensive or offensive use of force.”

Is Iran in breach of international law in its response?

Having come under attack, Iran is entitled to attack US and Israeli military bases in response, hence its foreign minister, Abbas Araghchi, telling NBC it was acting in self-defence. However, a hotel and airport have been hit in Dubai and Starmer has accused Iran of “indiscriminate” attacks against countries who were not parties to the conflict. Deliberate attacks on civilian targets are illegal under international humanitarian law. Attacks on countries which are not party to a conflict are also forbidden.