British Foreign Secretary Boris Johnson has said that “this confirmation cannot be ignored” after international chemical weapons watchdog OPCW published its report concluding that banned sarin gas was used in an attack on rebel-held Khan Sheikhoun in Syria earlier this year.

At least 90 people were killed in the 4 April incident in Idlib province, which several Western intelligence services believe was carried out by President Bashar al-Assad’s forces.

Images of children convulsing as they died from exposure to the toxins caused outrage around the world, and prompted the US to carry out a retaliatory military strike on a Syrian airbase.

Chemical attack '100 per cent fabrication', Assad says

“The UK's own assessment is that the Assad regime almost certainly carried out this abominable attack,” Mr Johnson wrote on Twitter.

“I urge our international partners to unite behind the need to hold those responsible for this atrocity to account.”

While the OPCW fact finding mission said in a statement accompanying the report on Thursday it had concluded that banned chemical agents were indeed used in the attack, it did not assign blame.

The organisation took pains to defend their methodology. Investigators did not visit to the scene, which was deemed too dangerous, but analysed samples from victims and interviewed witnesses.

A joint OPCW-UN panel is now tasked with trying to determine whether the Syrian government was responsible.

The Syrian government has repeatedly denied it has any chemical weapons stocks after surrendering its chemical arsenal to OPCW in 2013.

Syria: Man loses 25 family members in suspected chemical attack

Damascus, along with its Russian and Iranian allies, has said that the casualties were caused when a conventional air strike on an al-Qaeda weapons depot nearby caused an explosion, releasing the deadly gases.

Sarin is a nerve agent banned under international law which attacks the body’s central nervous system and causes nausea, difficulty breathing, blurred vision and loss of control of bodily functions.

The Khan Sheikhoun attack was the most deadly chemical incident in Syria since 2013. The US responded with a retaliatory “one off” missile strike on a government airbase near Homs - the first direct intervention taken by the US in the country in more than six years of civil war.

In pictures: US missile strike against Syria Show all 7 1 /7 In pictures: US missile strike against Syria In pictures: US missile strike against Syria The guided-missile destroyer USS Porter (DDG 78) launches a tomahawk land attack missile in the Mediterranean Sea AP In pictures: US missile strike against Syria The United States military launched at least 50 tomahawk cruise missiles at al-Shayrat military airfield near Homs, Syria, in response to the Syrian military's alleged use of chemical weapons in an airstrike in a rebel held area in Idlib province EPA In pictures: US missile strike against Syria Shayrat airfield in Syria Getty Images In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria President Donald Trump speaks at Mar-a-Lago in Palm Beach, Fla., after the US fired a barrage of cruise missiles into Syria in retaliation for this week's gruesome chemical weapons attack against civilians AP In pictures: US missile strike against Syria Syria's President Bashar al-Assad Reuters

The US State Department said in a statement issued Thursday night that the “The facts reflect a despicable and highly dangerous record of chemical weapons use by the Assad regime.”

Earlier this week the White House said it believed the Syrian government was planning another chemical attack after “abnormal activity” was observed at an airbase.