Soviet-developed nerve agent Novichok was used to poison Russian double agent Sergei Skripal and his daughter in Salisbury, Theresa May has revealed.
The prime minister said the substance used was a "military grade" nerve agent produced by Russia and there were only two possible explanations: either Moscow was behind the attack or it had lost control of its stockpile of the poison.
Novichok, the name for a family of nerve agents, was developed by the Soviet Union in the 1970s and 1980s and was said to be the world's most powerful nerve agent.
The name means "newcomer" in Russian, which signifies the huge breakthrough it made in chemical weapons development.
No one knew that Novichok existed until 1992, when a chemist whistleblower, Vil Mirzayanov, was jailed for revealing that the binary nerve agent had been secretly developed by the Soviets.
It was thought to be eight to 10 times as powerful as anything in America's arsenal at that time.
Hamish de Bretton-Gordon, a chemical weapons expert, told ITV News that Novichok makes other chemical weapons, such the VX nerve agent, which was used last year in the murder of North Korean Kim Jong-nam, seem "tame".
"Novichoks were a super-nerve agent developed by the Russians in the 80s and 90s," he said.
"They make VX and other nerve agents look quite tame in comparison."
A nerve agent is a toxic substance that disrupts signals in the nerves, causing debilitating side effects which can be fatal.
Nerve agents are a liquid, rather than a gas, however Novichok is typically administered as fine powder.
They were first discovered by accident in the 1930s, when scientists were attempting to find a more cost-effective pesticide.
It proved to be incredibly toxic and posed a risk to mammals, leading it into the hands of the German military, which crafted a weapon of war.
Russia came across such chemical agents for the first time when they swept into East Germany following the Second World War and took control of the plants where they were made.
In the UK, the ingredients required to create a nerve agent are carefully regulated.
It is thought Novichok has stayed in Russia since it was developed, giving the Government confidence that the country has either lost control of the poison or ordered the attack itself.