Boris Johnson and his French counterpart have urged the international community to make sure those responsible for the Syrian chemical weapon attack are held to account.
The Foreign Secretary and Jean-Marc Ayrault said the world had a "moral duty" to do more, stressing that it was "crucial for international peace and security" to uphold the ban on the use of the weapons.
The pair said there would be "no impunity" for those responsible for the attack, which they said was "highly likely" to have been carried out by Bashar Assad's regime.
They said two aircraft took off from Shayrat on April 4 - a "regime-run, Russian-supported airbase" which was later targeted by US missiles as Donald Trump ordered a retaliation for the atrocity.
"It seems highly likely that they bombed civilians... with murderous gas," the pair wrote in a strongly-worded Guardian editorial.
"What followed is too grim to describe. Painful, indiscriminate and long-lasting agony for babies, women and the elderly.
"Deaths by the score. Hundreds of wounded who will bear the scars for ever.
"It brings shame on the Syrian regime and its supporters. It brings shame on the world. How long can we endure this?"
Mr Johnson's alliance with Mr Ayrault is a sign that he is working to build support, having appeared isolated over the issue of sanctions at a gathering of G7 foreign ministers in Italy.
The Foreign Secretary is understood to have floated the idea of sanctions against the Syrian military commanders responsible for the attack, but Italian counterpart Angelino Alfano said there was "no consensus" on the issue.
Foreign Office sources said Mr Ayrault supported Mr Johnson's calls for evidence to be gathered to identify those responsible and then target them with sanctions or war crimes charges.
They supported the independent Organisation for the Prohibition of Chemical Weapons (OPCW) investigation into the attack, saying: "The international community has a moral duty to go further.
"It is our responsibility to uphold the strict prohibition of chemical weapons.
"It is our responsibility to ensure that those who conducted such chemical attacks are held accountable. It is crucial for international peace and security.
"In the name of the international community, the OPCW fact-finding mission will investigate the attack.
"The joint investigation mechanism, an independent UN-OPCW body, will then say who is responsible.
"We are confident in this process and we fully support it. There will be no impunity."
Syrian leader Mr Assad has dismissed the alleged chemical weapons attack in Khan Sheikhoun as a "fabrication" to justify the US missile strikes.
But the two foreign ministers said: "Regime denials should impress no one.
"Assad said it was the terrorists. But then Assad claims not to have barrel bombs, not to bomb civilians, not to torture, 'because it would be illogical'.
"No barrel bombs? Their images are everywhere. No attacks on civilians? Refugee camps in Turkey are replete with their victims.
"No torture? Amnesty International found that in the Saydnaya prison alone, 13,000 were slaughtered."
British scientists had analysed samples from the victims of the Khan Sheikhoun attack which tested positive for the sarin nerve agent or a sarin-like substance.
"When children are gassed, we can take no more abject lies," Mr Johnson and Mr Ayrault said.
"Our analysis indicates that two Syrian fast jets were in the vicinity of Khan Sheikhoun, and within range of likely impact sites.
"We believe that it is only the regime that has the capability to make such an attack. So it is highly likely that attack was carried out by the Assad regime."
The pair criticised Russia for vetoing a UN Security Council resolution on Syria and accused Moscow and Tehran of covering up details of Mr Assad's chemical weapons use.
But their article does not accuse Russia of being involved in the attack and sources said Mr Johnson had not pushed for sanctions against Russian individuals at the G7 meeting.