Russian Federation has described the reports of a chemical attack as a "provocation" created to justify Western intervention against the Syrian government, and accused militant rebels of fabricating it.
Opposition Labour Party leader Jeremy Corbyn said parliament should be consulted before May approves military action.
Mr. Macron said that he would respond "at a time of our choosing, when we judge it to be the most useful and the most effective".
US President Donald Trump, who said on Wednesday that missiles were "coming", has now tweeted that he "never said when".
Former French President Francois Hollande expressed on Thursday his support to the idea of taking military action as one of the possible responses to the alleged chemical attack on the Syrian city of Douma.
British Prime Minister Theresa May yesterday summoned her senior ministers to a special Cabinet meeting to discuss joining the U.S. and France in possible military action against Syria after a suspected poison gas attack on civilians.
Meanwhile, an global chemical weapons watchdog organization said Thursday that its team is en route to Syria and will begin work Saturday to investigate the attack, the Organization for the Prohibition of Chemical Weapons said.
President Emmanuel Macron of France has said he has "proof" that the Syrian government carried out chemical weapons attack on Douma.
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"We have proof that chemical weapons, at least chlorine, were used, and that they were used by the Bashar al-Assad regime", the French president said in an hour-long midday TV interview.
With increasing concerns about a US-Russia proxy war in Syria, Mr Macron insisted that "France will not allow an escalation or something that could damage the stability" of the region.
May recalled the ministers from their Easter holiday for the meeting at 3:30pm in Downing Street to discuss Britain's response to what she has cast as a barbaric attack that can not go unchallenged.
Kremlin spokesman Dmitry Peskov cautioned against "any steps which could lead to an escalation of tensions".
The White Helmets, an organization working inside Syria, claimed that 43 people were killed in the attack on Douma.
It remains hard to place an exact figure on the number of people killed and wounded Saturday in the rebel-held town of Douma, on the outskirts of Damascus.
The Syrian government has denied the allegations.
Douma was the last major rebel stronghold near Damascus.