Foreign Secretary Boris Johnson has denounced a suspected chemical weapons attack in Syria as a "war crime" and called for those responsible to be brought to justice.
It would be "unbelievable" to think that president Bashar Assad could play a role in the post-war government of the country if his regime is found to be to blame, Mr Johnson said.
Opposition activists claim that dozens of people died in the attack in a town in the northern province of Idlib, with the British-based Syrian Observatory for Human Rights putting the death toll at 58, including 11 children.
It was the third claim of a chemical attack in just over a week in Syria. There was no immediate comment from the government in Damascus on the alleged incident, which comes a day before a conference on the future of Syria co-hosted by Britain in Brussels.
Speaking in London, Mr Johnson - who will represent the UK at tomorrow's summit - said: "If this were proved to have been committed by the Assad regime, it would be another reason to think they are an absolutely heinous outfit.
"Bombing your own civilians with chemical weapons is unquestionably a war crime and they must be held to account.
"It is unbelievable to think that in the long term, Bashar Assad can play a part in the future of Syria, given what he has done to his people."
France's foreign minister Jean-Marc Ayrault called for an emergency United Nations Security Council meeting to discuss the "atrocious act".
And EU foreign policy chief Federica Mogherini described the reported attack as "awful", adding that Mr Assad's government "has the primary responsibility of protecting its people and not attacking its people".
The Syrian activists had no information on what agent could have been used in the assault on the town of Khan Sheikhoun, which they blamed on an air strike carried out either by the Syrian government or Russian warplanes.
Idlib province is largely opposition-controlled and is home to around 900,000 Syrians displaced from their homes elsewhere in the country.
Labour's shadow foreign secretary Emily Thornberry said: "This was a shocking and barbaric attack, and our thoughts are with all the victims and their loved ones.
"The use of chemical weapons by anyone cannot be tolerated, as the Syrian government itself accepted when it joined the Chemical Weapons Convention, and there must be no impunity for those found responsible."