Four Tornado jets fired Storm Shadow missiles "at a military facility -- a former missile base -- some 15 miles (24 kilometres) west of Homs, where the regime is assessed to keep chemical weapon precursors," the ministry said in a statement.

"Initial indications are that the precision of the Storm Shadow weapons and meticulous target planning have resulted in a successful attack," the ministry said.

"Very careful scientific analysis was applied to determine where best to target the Storm Shadows to maximise the destruction of the stockpiled chemicals and to minimise any risks of contamination to the surrounding area.

"The facility which was struck is located some distance from any known concentrations of civilian habitation, reducing yet further any such risk," it added.

Defence Secretary Gavin Williamson said in the statement that the strikes were a "legal and proportionate" response to the use of chemical weapons by the Syrian regime.