Mr Johnson said Mrs May will be making a statement in the House of Commons on Monday and it will give parliamentarians a chance to hold the executive to account.
The Organisation for the Prohibition of Chemical Weapons has been "working in close collaboration" with United Nations security experts "to assess the situation and ensure the safety of the team", it said.
Four Royal Air Force Tornado jets using Storm Shadow missiles had taken part in the attack on a military facility near Homs where it was assessed Syria had stockpiled chemicals, Britain's Ministry of Defence (MoD) said.
Syrian state media reported that internal security forces had entered Douma on Saturday and that the town would be secured within hours.
"This is not about intervening in a civil war".
"The government should do whatever possible to push Russian Federation and the United States to agree to an independent UN-led investigation of last weekend's horrific chemical weapons attack so that those responsible can be held to account", he added. "Yet... the global community is not convinced that a chemical attack took place in Douma, and wants to see the results of the ongoing OPCW investigation... to say that "there is no practicable alternative to the use of force if lives are to be saved" is hypocritical, to say the least", the statement says.
"This collective action sends a clear message that the worldwide community will not stand by and tolerate the use of chemical weapons", May said at a press conference, calling the military action "right and legal".
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"And the world said "enough" to the use of such weapons".
"We have sought to use every possible diplomatic channel to achieve this", the prime minister said.
Meanwhile Labour leader Jeremy Corbyn has said Parliament should have been given a vote ahead of the strikes.
"We are reassured that the military action is strictly targeted and limited in its goal".
Correspondents from the British and world press also asked the prime minister to explain what role can the Parliament play in making a decision to attack another sovereign country.
Prime Minister Justin Trudeau said in a statement that Canada stands with its allies and that it supports the decision " to take action to degrade the Assad regime's ability to launch chemical weapons attacks against its own people".
In a statement Friday night, U.S. President Donald Trump outlined strikes being carried out against the Syrian regime of Bashar al-Assad after a chemical weapon attack earlier in the week.