Airstrikes in Syria met international laws for military action on humanitarian grounds, according to No 10 documents setting out the legal case.
In a summary of the advice given by Attorney General Jeremy Wright to Prime Minister Theresa May, Downing Street pointed the finger at Moscow over its role in leading the UK and its allies to decide to launch an attack on Bashar Assad's regime.
It said international action to alleviate suffering caused by chemical weapons had been repeatedly blocked by Syria's allies.
Diplomatic action, sanctions, and the US strikes against the Shayrat airbase in April 2017 have failed to sufficiently degrade Syria's chemical weapons capability, the document added.
"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," the paper said.
UK ambassador to the UN Karen Pierce insisted the Government was "certain" of its legal case.
The No 10 paper said the UK met three demands under international law - that there is convincing evidence of extreme humanitarian distress, there is no practicable alternative to the use of force, and the action is necessary and proportionate - had been met.
It said: "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."