The poison used to target a former spy and his daughter was from the Russian-made nerve agent group novichok, the PM has said.
One chemical from the group, codenamed A-230, is between five and eight times more potent than VX, the nerve agent that was used to kill the half-brother of North Korean leader Kim Jong Un.
There are more than 100 formulations in the novichok family, all developed by the Soviet Union in the 1970s and 1980s.
Novichok is the Russian word for “newcomer”.
As with other nerve agents, the chemicals block acetylcholinesterase, an enzyme active in the nervous system.
The result is involuntary contraction of all muscles, leading to cardiac arrest and asphyxiation.
The agents are dispersed as an ultra-fine powder instead of a gas or vapour. They can be inhaled, ingested, or absorbed through the skin.
Significantly, several of the novichok chemicals are so-called binary weapons – with two non-toxic precursor chemicals that can be mixed prior to use.
This makes them safer to transport and handle.
One of these is novichok-5, derived from the highly potent A-230 chemical. Its precursor chemicals are ordinary organophosphate pesticides and can be legally made at agricultural chemical manufacturers.
According to Dr Vil Mirzayanov, who worked on the novichok programme in the Soviet Union until he fled to the US, the legitimate use of precursor chemicals was deliberate as it meant they were not placed on the Controlled List of the Chemical Weapons Convention.
Weapons expert Hamish de Bretton-Gordon also told one British media outlet that the novichok chemical was difficult to obtain, making it more likely that the poisoning was state-backed.
Theresa May told the Commons her Government has concluded it is “highly likely” Russia is responsible for the poisoning of Sergei Skripal and his daughter Yulia in Salisbury last Sunday.
The PM said the attack, using a “military-grade” nerve agent, fits a pattern of Russian aggression and said Russia’s ambassador to the UK has been summoned to explain what happened.
She said: “We will not tolerate such a brazen attempt to murder innocent civilians on our soil.”
Foreign Secretary Boris Johnson has told the Russian ambassador that Moscow must “immediately provide full and complete disclosure” of its novichok nerve agent programme to the Organisation for the Prohibition of Chemical Weapons.
Moscow has been given until the end of Tuesday to respond, May said.
If Russian involvement is proved, it will be considered an “unlawful use of force by the Russian state against the United Kingdom”, she added.
Skripal, 66, and his daughter Yulia, 33, remain in a critical condition more than a week after they were found slumped unconscious in Salisbury.