US President Donald Trump warned Russia on Wednesday of imminent military action in Syria, declaring that missiles "will be coming" and lambasting Moscow for standing by Syrian President Bashar al-Assad.
Russia has warned the West against attacking its Syrian ally, which is also supported by Iran, and says there was no chemical attack in the Syrian town of Douma.
May recalled the ministers from their Easter holiday for a special cabinet meeting in Downing Street later on Thursday to discuss Britain's response to what she has cast as a barbaric attack that cannot go unchallenged.
The BBC said May was ready to give the go-ahead for Britain to take part in action led by the United States without seeking prior approval from parliament. Downing Street spokesmen repeatedly declined to comment on that report.
"The chemical weapons attack that took place on Saturday in Douma in Syria was a shocking and barbaric act," May told reporters on Wednesday. "All the indications are that the Syrian regime was responsible."