Britain sold chemicals and components to Syria that ended up being used in the manufacture of the deadly nerve agent sarin, BBC Newsnight can reveal.
A leaked Foreign Office document says they were supplied in the mid-1980s.
Ministers will confirm within days that UK firms provided the materials and that Syria has admitted they played a role in its chemical weapons programme.
Syria's declarations to the Organisation for the Prohibition of Chemical Weapons (OPCW) name Britain as the sole supplier of dimethyl phosphate (DMP), trimethyl phosphate (TMP) and hexamine - three chemicals that Syria used in the production of sarin.
The phosphate chemicals are regarded as the building blocks of sarin and Newsnight understands British companies sold hundreds of metric tonnes of them to Damascus.
Syria's declarations have since been verified by a Whitehall audit of chemical sales by British companies.
Syrian crimes against humanity: enabled by the British-government-backed British arms industry.
The UK is at least helping to clean up the mess it helped make, contributing naval escorts and destruction facilities to the international program to destroy Syria's chemical weapons. But that's probably a small consolation to their victims.