Questions have been raised again about Novichok after German authorities said Russian opposition leader Alexei Navalny was poisoned with the nerve agent.

Mr Navalny, one of the most prominent critics of Russian President Vladimir Putin, fell ill on August 20 and was transferred to a Berlin hospital where tests found he had been poisoned.

The Soviet-era chemical came to international attention after the poisonings of five people in Wiltshire in 2018.

– What is Novichok?

Novichok is a group of nerve agents which are more potent and lethal than chemical weapons VX or sarin.

Alastair Hay, professor of environmental toxicology at the University of Leeds, said there are around 20 known chemicals with varying structures and toxicity involved.

Novichok, which means newcomer in Russian, was developed by the Soviet Union in the 1970s as a new kind of chemical weapon that would be harder to detect, more potent than existing nerve agents and exempt from the Chemical Weapons Treaty.

Very little is known about the manufacturing or distribution of the nerve agent.

– How does it work?

Like other nerve agents, Novichok attacks the nervous system and stops chemical messages getting around the body.

It causes the heart to slow down and airways to become constricted, leading to suffocation or brain damage.

Professor Hay said: “They do this by inhibiting a specific enzyme, acetylcholinesterase.

“The enzyme is crucial for regulating messages from nerve to muscle – and its inhibition leaves muscles in a sort of spasm. Muscles cannot contract and relax as they do normally.

“The nerve agents are extremely potent, with very tiny quantities sufficient to kill.”

– What happened in Salisbury?

On March 4 2018, former Russian spy Sergei Skripal and his daughter Yulia were found unconscious on a park bench in the city.

The pair were critically ill in intensive care following exposure to the then-unknown substance.

Days later, the substance was identified as Novichok and the case was treated as attempted murder after police said the pair were targeted deliberately.

The poisoning led to a decontamination effort in Salisbury, which took months to complete.

Wiltshire Detective Sergeant Nick Bailey was also taken to hospital after he became seriously ill.

He was believed to have come into contact with the deadly nerve agent when he and two colleagues searched Mr Skripal’s Salisbury home.

Ms Skripal was discharged on April 10 and her father was later discharged after two months of treatment.

Mr Bailey was discharged on March 22 and returned to work in January 2019.

– What happened in Amesbury?

Dawn Sturgess and her partner Charlie Rowley fell ill at his home in Amesbury, near Salisbury, on June 30.

Both were seriously ill and Ms Sturgess died in hospital on July 8.

It is believed they were exposed to the military-grade nerve agent from a perfume bottle discarded by those responsible for the attack on Mr Skripal and his daughter.

Mr Rowley survived the poisoning and later said Ms Sturgess had sprayed the substance on to her wrists believing it was perfume.

– How was it spread?

The Department for Environment, Food and Rural Affairs (Defra) said they believed the Novichok was delivered to the Skripals in a “liquid form”.

Those responsible for the attack are thought to have smeared the highly toxic chemical on a door handle at Mr Skripal’s Wiltshire home.

The highest concentration was found there, with eight other areas across the city potentially contaminated, Defra said.

– What impact did it have on UK-Russia relations?

The Salisbury incident sparked an international fallout, with then-home secretary Sajid Javid accusing the Russian state of using Britain as a “dumping ground for poison”.

On March 14, 10 days after the poisoning of the Skripals, then-prime minister Theresa May told MPs the UK would expel 23 Russian diplomats, calling the incident an “unlawful use of force by the Russian state against the UK”.

Three days later, Russia announced the expulsion of 23 UK diplomats and said it would shut down the British Council and British Consulate in St Petersburg.

Britain’s allies later announced that more than 100 Russian agents were being sent home from 22 countries, in what Mrs May called the “largest collective expulsion of Russian intelligence officers in history”.

– What about the suspects?

Two suspects were identified in the attack, known by their aliases Alexander Petrov and Ruslan Boshirov.

In September 2018, Mr Putin said there was “nothing criminal” about Petrov and Boshirov.

Downing Street insisted they were GRU officers “who used a devastatingly toxic illegal chemical weapon on the streets of our country”.

Petrov and Boshirov told Russian state-funded news channel RT that they travelled to the “wonderful” city in Wiltshire after recommendations from friends.

Boshirov told the broadcaster: “There’s the famous Salisbury Cathedral, famous not only in Europe but in the whole world.

“It’s famous for its 123-metre spire, it’s famous for its clock, the one of the first ever created in the world that is still working.”

They denied having any Novichok in their possession.