Theresa May warned that the West has the resolve to take further military action if chemical weapons are used again in the Syrian civil war.

The Prime Minister said the regime of Bashar al-Assad should be “under no doubt” that additional strikes would come if it tried to deploy the toxic weapons in the future.

After Britain joined the US and France in the first wave of strikes, the government released the legal case it says it has for them.

Directly linking the strikes to the suspected chemical weapons attack in Douma, it says that the continued use of chemical weapons by Bashar as-Assad's regime is "a war crime and a crime against humanity."

Read the legal case in full:

1.This is the Government’s position on the legality of UK military action to alleviate the extreme humanitarian suffering of the Syrian people by degrading the Syrian regime’s chemical weapons capability and deterring their further use, following the chemical weapons attack in Douma on 7 April 2018.

2.The Syrian regime has been killing its own people for seven years. Its use of chemical weapons, which has exacerbated the human suffering, is a serious crime of international concern, as a breach of the customary international law prohibition on the use of chemical weapons, and amounts to a war crime and a crime against humanity.

3.The UK is permitted under international law, on an exceptional basis, to take measures in order to alleviate overwhelming humanitarian suffering. The legal basis for the use of force is humanitarian intervention, which requires three conditions to be met:

(i) there is convincing evidence, generally accepted by the international community as a whole, of extreme humanitarian distress on a large scale, requiring immediate and urgent relief;

(ii) it must be objectively clear that there is no practicable alternative to the use of force if lives are to be saved; and

(iii) the proposed use of force must be necessary and proportionate to the aim of relief of humanitarian suffering and must be strictly limited in time and in scope to this aim (i.e. the minimum necessary to achieve that end and for no other purpose).

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4.The UK considers that military action met the requirements of humanitarian intervention in the circumstances of the present case:

(i) The Syrian regime has been using chemical weapons since 2013. The attack in Eastern Damascus on 21 August 2013 left over 800 people dead. The Syrian regime failed to implement its commitment in 2013 to ensure the destruction of its chemical weapons capability. The chemical weapons attack in Khan Sheikhoun in April 2017 killed approximately 80 people and left hundreds more injured. The recent attack in Douma has killed up to 75 people, and injured over 500 people. Over 400,000 people have now died over the course of the conflict in Syria, the vast majority civilians. Over half of the Syrian population has been displaced, with over 13 million people in need of humanitarian assistance. The repeated, lethal use of chemical weapons by the Syrian regime constitutes a war crime and a crime against humanity. On the basis of what we know about the Syrian regime’s pattern of use of chemical weapons to date, it was highly likely that the regime would seek to use chemical weapons again, leading to further suffering and loss of civilian life as well as the continued displacement of the civilian population.

(ii) Actions by the UK and its international partners to alleviate the humanitarian suffering caused by the use of chemical weapons by the Syrian regime at the UN Security Council have been repeatedly blocked by the regime’s and its allies’ disregard for international norms, including the international law prohibition on the use of chemical weapons. This last week, Russia vetoed yet another resolution in the Security Council, thwarting the establishment of an impartial investigative mechanism. Since 2013, neither diplomatic action, tough sanctions, nor the US strikes against the Shayrat airbase in April 2017 have sufficiently degraded Syrian chemical weapons capability or deterred the Syrian regime from causing extreme humanitarian distress on a large scale through its persistent use of chemical weapons. There was no practicable alternative to the truly exceptional use of force to degrade the Syrian regime’s chemical weapons capability and deter their further use by the Syrian regime in order to alleviate humanitarian suffering.

(iii) In these circumstances, and as an exceptional measure on grounds of overwhelming humanitarian necessity, military intervention to strike carefully considered, specifically identified targets in order effectively to alleviate humanitarian distress by degrading the Syrian regime’s chemical weapons capability and deterring further chemical weapons attacks was necessary and proportionate and therefore legally justifiable. Such an intervention was directed exclusively to averting a humanitarian catastrophe caused by the Syrian regime’s use of chemical weapons, and the action was the minimum judged necessary for that purpose.