"Horrific reports of chemical weapons attack in Idlib, Syria. Incident must be investigated and perpetrators held to account," Foreign Secretary Boris Johnson wrote on Twitter.
In a separate statement, he added: "This bears all the hallmarks of an attack by the regime, which has repeatedly used chemical weapons".
"If this is shown to be the work of the regime, it is further evidence of the atrocities perpetrated against the Syrian people over six years of appalling conflict," he said.
France has called for an emergency meeting of the UN Security Council over the incident and the EU has said Syrian President Bashar al-Assad bears "primary responsibility" for the incident.
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A senior Syrian security source, however, told AFP that the attack was "a false accusation".
Britain is co-hosting a Syria donors' conference in Brussels on Tuesday and Wednesday, and Johnson said he hoped it "steps up international assistance to the most vulnerable people affected by the conflict".
The Syrian government has repeatedly denied using chemical weapons in the civil war, which has killed 310,000 people since March 2011.