The Syrian regime used barrel bombs dropped by helicopter in a chemical weapon attack on its own people in the city of Douma, according to the US, France and Britain yesterday.Justifying the decision to mount air strikes against Syria the three governments laid out evidence that the regime had used chemical weapons in the attack earlier this month, which opposition activists, rescue workers and medics say killed more than 40 civilians.In a statement the White House said: “Multiple government helicopters were observed over Douma on April 7, with witnesses specifically reporting a Mi-8 helicopter, known to have taken off from the Syrian regime’s nearby Dumayr airfield, circling over Douma during the attack“Numerous eyewitnesses corroborate that barrel bombs were dropped from these helicopters, a tactic used to target civilians indiscriminately throughout the war. Photos of barrel bombs dropped in Douma closely match those used previously by the regime. These barrel bombs were likely used in the chemical attack.”The Prime Minister told a hastily arranged press conference at Downing Street, called within hours of the missile strikes, that all evidence pointed to the regime of Bashar al-Assad being responsible for the earlier attack on civilians. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. Children from Douma arrive in the Al-Bab district of Aleppo, Syria on April 11, four days after the chemical attack on their home city Credit:Anadolu Agency The World Health Organization said it had received reports from its local “health cluster partners” of 43 deaths related to symptoms consistent with “exposure to highly toxic chemicals”. “The Opposition does not operate helicopters or use barrel bombs. And reliable intelligence indicates that Syrian military officials co-ordinated what appears to be the use of chlorine in Douma on 7th April.”The Prime Minister also indicated there was other intelligence based evidence which she was unable to share with the public, saying: “I cannot tell you everything.”France has also said it has similar proof that “chemical weapons were used in the attack – at least chlorine – and that they were used by Bashar al-Assad’s regime.The Syrian government denies the claims, and its key ally Russia said it has “irrefutable evidence” that the incident was “staged” with the help of the UK. A picture released by the official Syrian Arab News Agency (SANA) on April 14, 2018, shows children standing next to a motorcycle in the Eastern Ghouta town of Douma, after Syrian government forces entered the last rebel bastionCredit:AFP Mrs May said: “Open source accounts allege that a barrel bomb was used to deliver the chemicals. Multiple open source reports claim that a Regime helicopter was observed above the city of Douma on the evening of 7th April. The attack on Douma, the last rebel-held town in the Eastern Ghouta region, started on April 6, after negotiations between the rebels and the government stalled.The bombardment continued into a second day, with dozens of people reportedly killed or injured by conventional munitions before the suspected chemical attack.Activists from the Violations Documentation Center (VDC), which records violations of international law in Syria, reported two separate incidents of bombs believed to contain toxic substances being dropped by the Syrian Air Force.The first was at 16:00 (13:00 GMT) on the Saada bakery on Omar Ibn Al-Khattab street in north-western Douma. The VDC said the second incident took place near Martyrs’ Square, at approximately 19:30.At 19:45, more than 500 patients – mostly women and children – were brought to medical facilities with symptoms suggesting exposure to a chemical agent, according to the Syria Civil Defence and the Syrian American Medical Society (SAMS).They showed signs of “respiratory distress, central cyanosis (blue skin or lips), excessive oral foaming, corneal burns, and the emission of chlorine-like odour”, according to a joint statement issued on Sunday.Rescue workers searching homes in the affected area also found the bodies of people with oral foaming, cyanosis, and corneal burns, the statement added. France’s intelligence services said they analysed witness accounts, photos and videos that appeared on specialized websites, in the press and on social media in the hours and days following the attack.They showed a massive influx of patients in health centres in Eastern Ghouta with symptoms consistent with exposure to a chemical agent. The VDC cited a rescue worker from the Syria Civil Defence as saying he smelt chlorine in the air after the strike, but that he could not determine its source.”We later discovered the bodies of people who had suffocated from toxic gases. They were in closed spaces, sheltering from the barrel bombs, which may have caused their quick death as no-one heard their screams,” he added.