The use of chemical weapons underlines the "cruel barbarity" of Bashar Assad's rule in Syria, Boris Johnson said as he urged the regime and its Russian backers to halt the bloodshed.
The Foreign Secretary said the chemical attacks were "horrific" and in breach of international law after an investigation found evidence of a third incident using chlorine gas.
The team from the United Nations and the Organisation for the Prohibition of Chemical Weapons found there was "sufficient evidence" to conclude that Syrian forces were responsible for one of the attacks in Qmenas in the Idlib region on March 16, 2015.
It said a device, dropped from a high altitude, "hit the ground and released the toxic substance that affected the population." Witnesses and hospital staff identified the smell and symptoms of chlorine gas.
An earlier report from the panel in August had previously blamed president Assad's government for using chlorine gas in two attacks and Islamic State, also known as Daesh, had used mustard gas in one incident.
Mr Johnson said: "This independent investigation has concluded that the Assad regime has used chemical weapons against the Syrian people on at least three occasions, and that Daesh has used such weapons at least once."
He said the attacks in Qmenas, along with the incidents in Talmenes and Sarmin,"demonstrate the cruel barbarity of the Assad regime in this conflict".
The use of chemical weapons by IS in Marea "shows their complete disregard for human rights, and international norms and values".
Mr Johnson said: "This report makes clear that the Assad regime has consistently failed to co-operate with the investigation.
"The use of chemical weapons is horrific, and a breach of international law and UN Security Council resolutions. It is crucial to hold those responsible to account.
"The regime and its backers, notably Russia, are inflicting the worst bombardment seen yet on Aleppo, and other towns and areas in Syria, causing severe humanitarian suffering. We are seeing heartbreaking scenes.
"The UK will continue to work with the international community to pursue an end to the regime's indiscriminate bombing campaign, full and sustained humanitarian access, an immediate and lasting ceasefire, and a return to the negotiating table."
The United States, Britain and France want the UN Security Council to impose sanctions on the Assad regime for using chemical weapons.
But Russia is expected to veto sanctions against Syria, whose forces have been supported by Vladimir Putin.
The Security Council is expected to discuss the new report on Thursday.