A powerful Russian nerve agent has been used on a Russian opposition leader, according to the German government.

Novichok nerve agents have been described as the most deadly ever made. They came to public prominence most recently in the wake of poisonings in the UK, where the Skripals and others were attacked.

But they have returned to prominence as the German government announced it had found Novichok in samples from Russian leader Alexei Navalny, who fell sick during a flight.

Use of nerve agents of this kind is banned under international conventions, because of the potency and potential horrible effects. They are developed in secret to get around those treaties and designed to avoid detection.

Novichok was first developed in the 1970s and Eighties by what was then the Soviet Union. The name means “newcomer” in Russian, and indicates the fact that when it was developed it marked a major breakthrough in the power of such chemical weapons.

That new potency meant that it was by some way the most powerful nerve agent in the world, and it continues to be thought one of the most deadly weapons available.

It can only be produced by highly specialised scientists, according to the researcher who helped develop it, and can only be used with intense supervision.

Novichok became famous in the Nineties, when a Soviet scientist called Vil Mirzayanov revealed that the country had secretly developed the powerful nerve gas, which was far more potent than anything in the US. Though the Soviet Union had been developing it for some time, it didn’t become known for as long as a decade after it was actually available, because it had been kept entirely secret.

It wasn’t clear how much of it the Soviet Union actually developed, though Mr Mirzayanov said Novichok was developed in much smaller amounts, meaning there might only be enough to kill several hundreds of thousands of people.

According to Professor Gary Stephens, pharmacology expert at the University of Reading: “This is a more dangerous and sophisticated agent than sarin or VX and is harder to identify. It causes a slowing of the heart and restriction of the airways, leading to death by asphyxiation.

“One of the main reasons these agents are developed is because their component parts are not on the banned list. It means the chemicals that are mixed to create it are much easier to deliver with no risk to the health of the courier.”

It works like all nerve agents, by overriding neurotransmitters in the body and shutting down the way it normally works. By forcing muscles to contract by attacking the nervous system, important parts of the body start to shut down – soon after someone comes into contact with such a nerve agent, the heart and diaphragm will shut down and death can result either through heart failure or suffocation.

Though it was developed to be as dangerous as possible when used, it was also designed so that it could be relatively easily transported. The nerve agent is made out of different “precursors”, which are relatively safe until they are mixed for use in a weapon.

That would presumably mean that the poison could transported relatively easily to Salisbury from Russia. It’s still not clear how such a poison would be able to make its way into the country.

As far as is known, the nerve agent has stayed in Russia since it was developed, which is what gives the Government such confidence that the country either used the poison itself or has lost control of it. But the Prime Minister did not speculate on the people or group behind the attack, or how it could have been carried out.

Nerve agents were first developed by the Nazis during the Second World War, and have been called by one expert “possibly some of the most dangerous things that humans have ever made, after the atom bomb”.

Just a drop of them is enough to entirely shut down a body, and countries including Russia, the US and the UK have been working to make them even more powerful since they were first introduced.

Prof Robert Stockman, professor of organic chemistry, University of Nottingham, said: “Fluoro phosphate-based nerve agents (such as Sarin, VX) work quickly – so in terms of half-lives within the body, this will not be very long (minutes, maybe hours).

“They react in stages with the nervous system – if an antidote can be administered within the first 24-48 hours, much of the effects can be reversed. However, the longer the victim goes before the antidote is administered, the agent’s effect on the nervous system ‘ages’ and the effects become irreversible.

“Within the environment, these agents react with water to degrade, including moisture in the air, and so in the UK they would have a very limited lifetime. This is presumably why the street in Salisbury was being hosed down as a precaution – it would effectively destroy the agent.”

He added: “In terms of their effect on the body, they interfere with the nervous system, rendering the parts affected inoperable – thus the uncontrollable muscle spasms, the difficulty breathing – the nervous system tells these muscles when and how to move, and interfering in this causes the spasms. The effects of the nervous agent can only be removed with an antidote given in time, or irreparable damage will be done.”