The Syrian National Coalition of Revolutionary and Opposition Forces, as well as a number of Western states, accused the Syrian government troops of carrying out the April 4 attack at Khan Shaykhun, near Idlib, but Damascus refuted the allegations and a Syrian army source told Sputnik they did not possess chemical weapons.
But on Friday (June 30) Mr. Johnson insisted the Assad regime was responsible and urged international action against Damascus.
"The exact responsibility for dropping the sarin will now go to a joint investigative mechanism to be confirmed but I've got absolutely no doubt that the finger points at the Assad regime," said Mr. Johnson, who has a reputation in Britain for bumbling and ineptitude.
President Assad had "really embarrassed Moscow by allowing his generals" to deploy chemical weapons, claimed Mr. Johnson.
"That is one of the objectives that we're trying to achieve: to drive a wedge between the Russians and the Iranians and between the Russians and Assad," he added.
Last week he was widely mocked on social media for a "car crash" radio interview in which he struggled to explain what his own government would do in the Queen's Speech to tackle "burning injustices".
Diplo source:terrorists encouraged by US threats vs Syria,planning to set up chemical incident to provoke US intervention. Info passed to US pic.twitter.com/wAXTFhiwOt— Russian Embassy, UK (@RussianEmbassy) 30 June 2017
Mr. Johnson did not specify what sort of international action he wanted to see taken.
It is not the first time Mr. Johnson had pointed the finger at President Assad. In April he said Damascus "almost certainly" carried out the attack.
The Organization for the Prohibition of Chemical Weapons said on Friday it had confirmed sarin was to blame for the Idlib deaths, but it did not confirm who was to blame for the incident.
The Russian Defense Ministry said on April 5 that the airstrike near Khan Shaykhun by the Syrian Air Force hit a terrorist warehouse that stored chemical weapons slated for delivery to Iraq, and called on the UN Security Council to launch a proper investigation into the incident.
Russian President Vladimir Putin said April 6 that groundless accusations, which Damascus denied, were unacceptable before the investigation into the matter had been carried out.
However, the incident was used as pretext for a US missile strike against the Ash Sha'irat airbase carried out late on April 6.
US President Donald Trump characterized the strike as a response to the alleged use of chemical weapons by the Syrian government troops while Russian Foreign Minister Sergei Lavrov said it was a violation of the international law.
Iranian Supreme Leader Ayatollah Ali Khamenei described the US missile strike against the Syrian airfield as a strategic mistake.
Trump's decision to launch a cruise missile strike on a Syrian airbase sparked a major diplomatic crisis.
Earlier this week the White House said it had evidence the Assad regime was preparing another chemical attack and warned it would "pay a heavy price" if it went ahead.
It also emerged this week that President Trump authorized the strike without the approval of the US Congress.