Boris Johnson has made a fresh appeal for international action against Bashar Assad after a chemical weapons watchdog ruled an attack in Syria that killed more than 90 people used sarin nerve gas.
The Foreign Secretary said the Syrian president's brutal regime was behind the atrocity and insisted the findings of the probe "cannot be ignored".
In the wake of the massacre, Russia blocked a British-backed bid at the UN Security Council to condemn the attack and Mr Johnson failed to secure agreement on targeted sanctions at a meeting with G7 counterparts.
Mr Johnson urged international leaders to now "unite behind the need to hold those responsible for this atrocity to account".
The Organisation for the Prohibition of Chemical Weapons (OPCW) found the deadly attack on rebel-held Khan Sheikhoun "can only be determined as the use of sarin, as a chemical weapon" after it analysed samples from victims and survivors.
Its report did not conclude who was responsible for the atrocity but the United Nations will now use the findings in its own assessment.
Mr Assad dismissed the attack as a "fabrication" to justify US missile strikes, and Russian president Vladimir Putin, the Syrian leader's key international ally, claimed it was "a provocation" staged "by people who wanted to blame" the dictator.
Mr Johnson said: "The Organisation for the Prohibition of Chemical Weapons has now confirmed that sarin nerve gas was indeed used in a chemical attack on the town of Khan Sheikhoun in northern Syria on April 4.
"This confirmation cannot be ignored. The UN-OPCW joint investigative mechanism will now work to identify who was responsible.
"As I have said previously, the UK's own assessment is that the Assad regime almost certainly carried out this abominable attack.
"I urge our international partners to unite behind the need to hold those responsible for this atrocity to account."