Direct proof of use
The United Kingdom Ministry of Defence identified Storm Shadow use in the 2011 Syrian Civil War during the April 14, 2018 coalition strike phase. It said four Royal Air Force Tornado GR4 aircraft, supported by four RAF Typhoon FGR4 aircraft, launched eight Storm Shadow missiles at the Him Sinshar chemical-weapons storage site west of Homs.
A U.S. Central Command briefing independently listed eight British Storm Shadow missiles among the 22 weapons assigned to the Him Shinshar chemical-weapons storage facility. The same briefing said British forces flew Tornado and Typhoon aircraft and launched eight Storm Shadow missiles as part of the wider U.S., UK, and French operation.
Sources: UK RAF Syria Strike, CENTCOM Syria Strike Briefing
Timeline
On April 7, 2018, the Douma chemical-weapons allegation became the immediate trigger cited by the United Kingdom, United States, and France for later strikes against Syrian government chemical-weapons-related facilities. The OPCW Fact-Finding Mission later reported reasonable grounds that a toxic chemical containing reactive chlorine was used in Douma, while noting that its mandate did not identify who was responsible.
At 0200 UK time on April 14, 2018, British forces joined the U.S. and French operation. The UK account placed the Storm Shadow launches against Him Sinshar, about fifteen miles west of Homs, and the U.S. briefing placed the Storm Shadow allocation within a 105-weapon coalition strike package against three Syrian chemical-weapons-related targets.
Sources: UK RAF Syria Strike, CENTCOM Syria Strike Briefing, OPCW Douma FFM Report
Narrative
Storm Shadow's documented Syrian Civil War role was a standoff precision strike weapon used by UK aircraft, not a weapon transferred to a Syrian belligerent. The available official accounts identify the user as the Royal Air Force, the launch aircraft as Tornado GR4s with Typhoon support, and the target as the Him Sinshar chemical-weapons storage site west of Homs.
The U.S. account describes the target set as coalition-wide: U.S. Tomahawk missiles, British Storm Shadows, and French missiles were all assigned to the Him Shinshar storage facility, while other coalition weapons struck the Barzah research center and the Him Shinshar bunker facility. This places Storm Shadow in a single, limited strike episode within the wider Syrian Civil War rather than in sustained theater use.
Sources: UK RAF Syria Strike, CENTCOM Syria Strike Briefing