- Video report by ITV News Political Editor Robert Peston
The Prime Minister told MPs on Monday that the decision to launch airstrikes against Syria without a vote in Parliament "not just morally right but also legally right."
Theresa May hit back at suggestions from Labour leader Jeremy Corbyn that the UK was following orders from the the US by joining action against the Syrian regime.
Mrs May told parliament: "Let me be absolutely clear, we have acted because it is in our national interest to do so.
"It is in our national interest to prevent the further use of chemical weapons in Syria and to uphold and defend the global consensus that these weapons should not be used.
"For we cannot allow the use of chemical weapons to become normalised within Syria, on the streets of the UK or elsewhere.
"So we have not done this because President Trump asked us to do so. We have done it because we believed it was the right thing to do and we are not alone."
She said military action was "not just morally right but also legally right" to "alleviate further humanitarian suffering".
She added: "This was not about intervening in a civil war and it was not about regime change. It was about a limited, targeted and effective strike that sought to alleviate the humanitarian suffering of the Syrian people by degrading the Syrian regime's chemical weapons capability and deterring their use."
Mrs May said the Western allies had no other option to try to curb the "abhorrent" use of chemical weapons.
"We've explored every possible diplomatic channel to do so but our efforts have been repeatedly thwarted," she said.
The PM said that British RAF jets joined French and US allies to target three areas in Damascus and Homs on Saturday including science sites, military facilities and equipment storage spaces.
She said the action was taken to cause minimal civilian casualties and that no Russian military or equipment were lost.
No group in Syria, apart from the government, could have carried out the April 7 attack in Douma, she said.
She pointed to previous diplomatic efforts via the UN Security Council being blocked by Russia, adding: "Regrettably we had no choice to conclude that diplomatic action on its own was not going to work."
Russia and Syria were preventing OPCW inspectors from travelling to Douma, she added.
"We support strongly the work of the OPCW fact-finding mission that is currently in Damascus.
"But that mission is only able to make an assessment of whether chemical weapons were used.
"Even if the OPCW team is able to visit Douma to gather information to make that assessment - and they are currently being prevented from doing so by the regime and the Russians - it cannot attribute responsibility.
"This is because Russia vetoed in November 2017 an extension of the joint investigatory mechanism set up to do this."
The government is not legally bound to seek Parliament's approval for military strikes, though it is customary to do so.
Mrs May said: "Governments of all colours have long considered that military action on an exceptional basis, where necessary and proportionate, and as a last resort to avert an overwhelming humanitarian catastrophe, is permissible under international law."
Labour leader Jeremy Corbyn branded the decision "legally questionable" quoting the United Nations Charter which states action must be in self defence or be authorised by the UN Security Council.
Describing the chemical attack in Douma as "horrific", Mr Corbyn faced shouts of "shame" from Tory MPs as he told the Commons: "This statement serves as a reminder that the Prime Minister is accountable to this Parliament, not to the whims of the US President."
Speaker John Bercow had to intervene to calm MPs, telling them Mr Corbyn must be afforded the same "respectful quiet" atmosphere given to Mrs May.
He urged Mrs May to publish the advice she had received from the Attorney General in relation to the airstrikes.
He also raised concerns over the use of banned cluster bombs and white phosphorous by Saudi Arabia as he raised humanitarian concerns over Yemen.
Mr Corbyn asked: "Will the Prime Minister commit today to ending support to the Saudi bombing campaign and arms sales to Saudi Arabia?"
Following Mrs May's speech, the Russian embassy in London accused Theresa May of making "misleading" claims about the OPCW.
"It's Russia and Syria who invited OPCW experts and have been working hard to ensure speedy arrival," the embassy said on its Twitter feed.
It also accused the Prime Minister of basing her attack on Syria on "staged" photos and films.
It claimed that 71 of the 103 missiles fired by the allies had been intercepted by Syrian air defences.
Mrs May spent three hours and 10 minutes at the despatch box in the Commons delivering her Syria statement and answering questions from MPs.
Speaker John Bercow said 140 backbench MPs questioned the PM.