The nerve agent used on Sergei and Yulia Skripal has been traced to a specific Russian lab, according to media reports.
The Times newspaper named Shikhany military base as the source of the chemical.
Britain has not officially identified the location, but has said it has strong intelligence linking the chemical — called Novichok — to Russia.
Russia has denied any involvement in the poisoning at all.
The nerve agent used to poison ex-spy Sergei Skripal was made in a secretive military science facility in southwestern Russia, according to intelligence cited by British media.
The Times newspaper reported that British officials have linked the chemical used on Skripal and his daughter Yulia to Shikhany military base, Russia's leading chemical research facility.
It is the most specific accusation yet to Russian culpability in the poisoning, which it has consistently denied.
Shikhany, around 600 miles from Moscow, is part of a complex that is completely closed to outsiders. The nearest major settlement is Volsk, a city of 65,000 people on the Volga river.
The source of the military-grade poison was revealed in a British intelligence briefing for its allies, and was used to persuade world leaders that Russia was responsible for the attack, The Times said.
After lobbying from Britain, an alliance of more 20 countries expelled more than 100 diplomats in retaliation for the attack.
What is not in doubt is that Novichok was also first developed in Shikhany during the Cold War.
Little else is known about the secretive military site because nobody can enter it without a permit. According to The Times, Shikhany is home to the Novichok chemical lab as well as cafés, groceries, hospitals, and schools.
A satellite image (at the top of this article) of Shikhany-2 — the closed, military part of the town — shows dozens of buildings lined up in a row, some of which have parking lots. The exact location of the chemical weapons lab within the facility is not known.
Hamish de Bretton Gordon, the former commanding officer of the UK's Chemical, Biological, Radiological and Nuclear Regiment, told The Times he had seen the intelligence in question, and backs its conclusion.
He told The Times: "The intelligence Britain has clearly points to Russia and Shikhany. No doubt the Russians are scrubbing it down as we speak."
Business Insider asked the British government to comment on the record, but has yet to receive a response.