Chemicals supplied by British firms to Syria in the early 1980s are likely to have been used to make nerve agent sarin, William Hague has said.
The Foreign Secretary said a review of records showed a number of companies exported substances to the country between 1983 and 1986 that had legitimate uses in producing plastics and pharmaceuticals, and were not restricted under international or UK law.
"However, they can also be used in the production of sarin," Mr Hague told MPs in a written statement.
"DMP (chemical dimethyl phosphite) and TMP (trimethyl phosphite) can also be used for the production of the nerve agent VX.
"That is why the export of such goods is strictly prohibited under the UK export regime introduced since the 1980s and progressively strengthened.
"From the information we hold, we judge it likely that these chemical exports by UK companies were subsequently used by Syria in their programmes to produce nerve agents, including sarin.
"Some of the companies involved no longer exist.
"Furthermore, some of the chemicals in question may have been sourced by a UK chemical trader, rather than produced in the UK.
"The review of our records also confirmed an export of ventilation fans by a UK company to Syria in 2003.
"The fans were not controlled goods.
"Following an enquiry by the exporter, officials considered the export under licensing procedures, and insufficient grounds for refusal were found.
"Syria appears to have diverted these fans for use in a chemical weapons facility.
"In the early 1980s, the exported chemicals were not subject to any international or UK export controls.
"However, knowledge of these exports, and growing concerns that Iraq under Saddam Hussein was developing a chemical weapons capability, helped prompt the introduction of tighter controls, both in the UK and internationally."
Mr Hague said Britain was "playing its full part" in the international effort to eliminate Syria's programme.
"As the House is already aware, the UK is accepting 150 tonnes of B precursors from the Syrian chemical stockpile for destruction," he said.
"I can today also inform the House that in addition to those chemicals, a further 50 tonnes of the industrial chemicals hydrogen chloride and hydrogen fluoride will also be destroyed in specialised commercial facilities in the UK.
"We expect the ship transporting all these chemicals to arrive in the UK next week."
A report by the UN last year found that Bashar Assad's regime had deployed chemical weapons against rebel forces.
In one particular incident in Damascus they were said to have been used "on a relatively large scale, resulting in numerous casualties, particularly among civilians and including many children".
Damascus has always denied using chemical weapons during recent years of fighting, blaming opposition forces.