Boris Johnson has branded President Bashar Assad an "arch-terrorist" and warned the US could launch new strikes against Syria.
Writing in the Sunday Telegraph the Foreign Secretary also told Moscow it still has time to be on the "right side of the argument" in the Syrian conflict.
Mr Johnson was widely criticised for failing to get the G7 nations to back his bid for new sanctions against senior Russian and Syrian figures in the aftermath of the chemical weapons attack on civilians in Idlib province which left at least 86 people dead, including 30 children.
In a column for the newspaper, Mr Johnson said: "Assad uses chemical weapons because they are not only horrible and indiscriminate. They are also terrifying.
"In that sense he is himself an arch-terrorist, who has caused such an unquenchable thirst for revenge that he can never hope to govern his population again.
"He is literally and metaphorically toxic, and it is time Russia awoke to that fact. They still have time to be on the right side of the argument."
Mr Johnson continued that it was "highly likely" Assad was responsible for the suspected Sarin gas attack, and insisted the US "could of course strike again".
"British scientists have analysed samples from the victims of the attack. These have tested positive for Sarin or a Sarin-like substance.
"The UK, the US and all our key allies are of one mind: we believe that this was highly likely to be an attack by Assad, on his own people, using poison gas weapons that were banned almost 100 years ago, under the 1925 Geneva protocol."
Mr Johnson said the chemical assault had changed the West's stance on Syria.
"Let us face the truth: Assad has been clinging on. With the help of Russians and Iranians, and by dint of unrelenting savagery, he has not only recaptured Aleppo. He has won back most of 'operational' Syria.
"Before the chemical weapons attack on April 4, the West was on the verge of a grim consensus - that it would be more sensible to concentrate on the fight against the terrorists of Daesh, and to accept reluctantly that removing Assad - though ultimately essential - should await a drawn out political solution."