Boris Johnson has called President Bashar al-Assad's denials that he did not order a chemical weapon attack on Idlib that killed more than 80 people, "absurd".
In his first interview since the strike and the subsequent US air assault, the Syrian president said reports the Syrian government carried out the attack were "100% fabrication".
Speaking to Agence France-Presse (AFP), al-Assad said the government had given up its chemical arsenal in 2013 and questioned the authenticity on video footage of the aftermath of the attack.
Writing in the Sunday Telegraph, Johnson said the defence was "absurd and mendacious" and described the premier as an "arch-terrorist" and "literally and metaphorically toxic".
Referring to the West's failure to act in 2013, the foreign secretary said one of the reasons why al-Assad approved the strike was because he could "get away with it.
"That is why Assad launched his sickening strikes, on April 4, with what looks like such insouciance," he wrote. "He thought that he could simply get away with it again and that his actions would be lost in the general fatigue over Syria.
"He allowed his air force to use poison gas because he has contempt for his own people and because he no longer believes in the willingness of the world to stand up to him."
Turning his attention to Russia, with whom Johnson pulled out of a meeting with after the US air strike of the Shayrat airbase, he said the Russians had the influence to put into place a ceasefire and end al-Assad's "tyranny".
The column came hours after Russia's foreign minister Sergei Lavrov said the chemical weapons attack may have been staged. Speaking at a conference in Moscow on Saturday (15 April), he said there were inconsistencies in the evidence that has emerged so far.