- Video report by ITV News Political Correspondent Daniel Hewett
Defence Secretary Sir Michael Fallon has said Russia is responsible for "every civilian death" in Bashar Assad's chemical weapons attack on his own people.
Writing in the Sunday Times, he said Vladimir Putin's Kremlin was to blame "by proxy" for the attack that killed 87 people, including 20 children, because it was the Syrian president's "principal backer".
Mr Johnson was due to visit Russia on Monday for talks with counterpart Sergey Lavrov, which would have been the first visit by a UK foreign secretary in more than five years.
The defence secretary's newspaper article saw him add his support for a long-term solution to the civil war in which Assad must no longer be president.
"Someone who uses barrel bombs and chemicals to kill his own people simply cannot be the future leader of Syria," Sir Michael wrote.
The suspected sarin nerve agent strike on the rebel-held town of Khan Sheikhoun on Tuesday led to the retaliation airstrikes by US president Donald Trump, which were in soon condemned by Russia.
The defence secretary said Moscow must be part of the solution to ending the "needless suffering" of Syrian civilians.
"But Assad's principal backer is Russia. By proxy Russia is responsible for every civilian death last week," Sir Michael wrote.
"If Russia wants to be absolved of responsibility for future attacks, Vladimir Putin needs to enforce commitments, to dismantle Assad's chemical weapons arsenal for good, and to get fully engaged with the UN peacekeeping progress."
Sir Michael also reiterated his support of Mr Trump's cruise missile strike in the early hours of Friday.
"By sending Tomahawk missiles to attack the airfield, aeroplanes and equipment believed to be involved, it has sent a strong signal to the Syrian regime to think twice before using gas in the future," he wrote.