"All the evidence I have seen suggests this was the Assad regime... using illegal weapons on their own people," Johnson said as he arrived for a Syria aid conference in Brussels.
"What it confirms to everybody is that this is a barbaric regime which has made it impossible for us to imagine them (having) authority over Syria after this conflict," he added.
The Brussels conference, co-chaired by the EU and UN, is a follow up to last year's London meeting which raised $11 billion (10 billion euros) for humanitarian aid programmes in the devastated country.
It is also meant to support UN-sponsored peace talks in Geneva where mediator Staffan de Mistura has reported some very modest progress in solving a conflict which has claimed more than 320,000 lives and displaced most of the Syrian population.
Assad's future role is a key sticking point - the rebels and their international backers demand that he must step down.
EU foreign affairs chief Federica Mogherini urged the international community to press ahead with the peace talks, which were made even more urgent after Tuesday's suspected chemical weapons attack.
"We need to give a push, a strong push to the political talks in Geneva. We have to unite the international community behind these negotiations," Mogherini said.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)