Bombing Syria was in Britain’s national interest and has had strong international support, Theresa May will insist as she tells Parliament why she ordered the attack.
The Prime Minister is expected to face angry MPs after launching military action without securing the support of the Commons.
But she will say the UK joined the United States and France in co-ordinated strikes following the chemical weapons attack in Douma to “alleviate further humanitarian suffering”.
Mrs May will ask for an emergency debate to allow more time for discussion in a nod to the fury among MPs at not being consulted, but she is not offering a vote.
“There is broad based international support for the action we have taken,” she will say.
It comes amid reports that Russia has launched a “dirty tricks” campaign in response to the strikes.
Whitehall sources have confirmed a 20-fold increase in “disinformation” spread by Kremlin-linked social media “bot” accounts since the strikes, according to The Daily Telegraph.
Foreign Secretary Boris Johnson said the UK has to “take every possible precaution” to prepare for possible revenge cyber attacks on targets like the NHS and electrical facilities.
Jeremy Corbyn has called for the introduction of a War Powers Act to stop governments launching military action in most circumstances without the backing of the Commons.
The Labour leader said chlorine has been used by “a number of parties in the conflict” in Syria as a weapon and questioned the legality of the airstrikes.
Mrs May spoke in Downing Street in the hours after the blitz and insisted the action was a limited and targeted strike to degrade and deter the Syrian government.
But she also drew a link with the nerve agent attack on Russian former spy Sergei Skripal and his daughter Yulia in Salisbury.
The Prime Minister will tell MPs on Monday that the strikes were in the national interest because the use of chemical weapons cannot be normalised, including in the UK.
She will say: “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 – either within Syria, on the streets of the UK or elsewhere.”
The Prime Minister spent Saturday evening speaking to world leaders to explain why Britain had joined forces with France and the US and will insist the three nations are “not alone” in believing it was the “right thing to do”.
Germany, Italy, Canada, Australia and European Council President Donald Tusk have “all have expressed their support for the actions that Britain, France and America have taken”, the PM will add.
United Nations Security Council-mandated inspectors have probed previous attacks and decided Bashar Assad’s regime was responsible four times, MPs will be told.
“We are confident in our own assessment that the Syrian regime was highly likely responsible for this attack and that its persistent pattern of behaviour meant that it was highly likely to continue using chemical weapons,” Mrs May will add.
“Furthermore, there were clearly attempts to block any proper investigation, as we saw with the Russian veto at the UN earlier in the week.
“And we cannot wait to alleviate further humanitarian suffering caused by chemical weapons attacks.”
Four Royal Air Force Tornado GR4s joined the co-ordinated missile strikes at 2am on Saturday, launching Storm Shadow missiles at a base 15 miles west of Homs.
The United States is preparing to impose sanctions on Russia for “covering up” the actions of the Assad regime.