Georgia's authorities used a World War One-era chemical weapon to quell anti-government protesters last year, evidence gathered by the BBC suggests.

"You could feel [the water] burning," one of the protesters said of water cannon turned on him and others on the streets of the capital Tbilisi. A sensation, he said, which could not immediately be washed off.

Demonstrators against the Georgian government's suspension of its European Union accession bid have complained of other symptoms too - shortness of breath, coughing, and vomiting that lasted for weeks.

The BBC World Service has spoken to chemical weapons experts, whistleblowers from Georgia's riot police, and doctors, and found the evidence points to the use of an agent that the French military named "camite".

The Georgian authorities said our investigation findings were "absurd" and the police had acted legally in response to the "illegal actions of brutal criminals".

Camite was deployed by France against Germany during World War One. There is little documentation of its subsequent use, but it is believed to have been taken out of circulation at some point in the 1930s, because of concerns about its long-lasting effects. CS gas - often referred to as "tear gas" - was used as a replacement.

Konstantine Chakhunashvili was one of those who gathered outside Georgia's parliament in Tbilisi during the first week of protests - which began on 28 November 2024. Demonstrators were incensed by the ruling party's announcement that it was pausing EU accession talks. The goal of EU membership is enshrined in Georgia's constitution.

Georgia's police responded with a variety of riot-control measures including the use of water cannon, pepper spray and CS gas.

Dr Chakhunashvili, a paediatrician who was among those sprayed by the cannons, and who has taken part in many of the demonstrations, said his skin felt like it was burning for days, and the sensation couldn't be washed away. In fact, he said, "it was worse when trying to wash it off".

Dr Chakhunashvili wanted to find out if others had suffered similar effects. So he appealed, via social media, for those also targeted by crowd control measures during the first week of the demonstrations to fill out a survey. Nearly 350 people got in touch, and almost half said they had suffered one or more side-effects for more than 30 days.

These long-term symptoms ranged from headaches, to fatigue, coughs, shortness of breath and vomiting.

His study has since been peer reviewed and has been accepted for publication by Toxicology Reports, an international journal.

Sixty-nine of those surveyed by Dr Chakhunashvili were also examined by him and found to have "significantly higher prevalence of abnormalities" in the electrical signals in the heart.

Dr Chakhunashvili's report echoed the conclusion that local journalists, doctors, and civil rights organisations had come to - that the water cannon must have been laced with a chemical. They had called on the government to identify what had been used, but the Ministry of Internal Affairs - responsible for the police - refused.

Several high-level whistleblowers connected to the Special Tasks Department - the official name of Georgia's riot police - helped the BBC determine this chemical's likely identity.

A former head of weaponry at the department, Lasha Shergelashvili, believes it is the same compound he was asked to test for use in water cannon in 2009.

The effects of that product, he says, were unlike anything he had previously experienced. He found it difficult to breathe after standing close to where it had been sprayed, and he and the 15-20 colleagues who tested it with him could not easily wash it off.

"We noticed that the effect was not wearing off, as is the case for [regular] tear gas. Even after we washed our faces with water, and then with a special solution of baking soda and water, which was prepared in advance, we still couldn't breathe freely."

Mr Shergelashvili says that as a result of his tests, he recommended against the chemical's use. But he says the water cannon vehicles were nevertheless loaded with it - and that this remained the case at least up until 2022, when he quit his job and left the country.

Speaking from his new home in Ukraine, he tells the BBC that when watching footage of the protests last year, he immediately suspected that demonstrators were being subjected to the same chemical.

Colleagues he has remained in touch with, and who are still in post, have also told him this is the case, he adds.

And the BBC spoke to another former high-level police officer who confirmed that whatever was loaded into the water cannon vehicles when Mr Shergelashvili was in position was the same compound deployed in the protests of November-December 2024.

• Viewers in the UK can see more on Global Eye at 19:00, 1 December on BBC Two
• Outside the UK, watch the film on BBC World Service YouTube

When Mr Shergelashvili was asked if the product he tested could have just been CS gas - which irritates the eyes, skin and respiratory system, but only temporarily - he said it appeared to be far stronger than that.

"I cannot name an example or compare it with anything [else]," he said, adding it was "probably 10 times" stronger than more conventional riot-control agents.

"For example, if you spill this chemical on the ground, you won't be able to stay in that area for the next two to three days, even if you wash it off with water."

Mr Shergelashvili does not know the name of the chemical he was asked to test.

But the BBC managed to obtain a copy of the inventory of the Special Tasks Department, dated December 2019.

We discovered it contained two unnamed chemicals. These were simply listed as "Chemical liquid UN1710" and "Chemical powder UN3439", along with instructions for how they should be mixed.

We wanted to check whether this inventory was authentic, so we showed it to another former high-ranking police officer from the riot police who confirmed it seemed genuine. He identified the two unnamed chemicals as those likely to have been added into the water cannon.

Our next step was to work out what these chemicals were.

UN1710 was easy to identify as this is the code for trichloroethylene (TCE), a solvent that enables other chemicals to dissolve in water. We then had to work out which chemical it was helping to dissolve.

UN3439 was much harder to identify because it is an umbrella code for a whole range of industrial chemicals, all of which are hazardous.

The only one of these we found to have ever been used as a riot-control agent is bromobenzyl cyanide, also known as camite, developed by the Allies for use in World War One.

We asked Prof Christopher Holstege, a world leading toxicology and chemical weapons expert, to assess whether our evidence pointed to camite being the likely agent used.

Based on the results of Dr Chakhunashvili's study, victim testimony, the riot police inventory, and Mr Shergelashvili's account of the chemical tests, Prof Holstege believes that this is the case.

"Based on the available evidence… the clinical findings reported by both those exposed and by other witnesses are consistent with bromobenzyl cyanide."

He ruled out the likelihood of the symptoms being caused by more conventional crowd control measures, such as CS gas, which was also being deployed by Georgia's riot police last year.

"The persistence of the clinical effects… is not consistent with the typical agents used for crowd dispersal, such as CS," he said.

"I've never seen camite being utilised in modern society. Camite is markedly irritating [and] persistent with its irritation."

He speculated it would have been used because it would act as a strong deterrent.

"It would keep people away for a long time. They couldn't decontaminate [themselves]. They would have to go to the hospital. They would have to leave the area. If that is indeed the case - that this chemical has been brought back - that is actually exceedingly dangerous."

Camite was briefly used as a riot control agent by the American police post-World War One, but was abandoned after safer options such as CS gas were invented.

Under international law, police forces are allowed to use chemicals as crowd-control agents as long as they are considered proportionate and have only short-term effects.

Given there are safer and more conventional riot-control agents available to police, an obsolete and more potent agent could be classed as a chemical weapon, according to weapons experts consulted by the BBC.

Our findings were concerning, said UN Special Rapporteur on Torture, Alice Edwards. Ms Edwards has previously written to Georgia's government regarding allegations of police violence and torture during the protests.

The lack of strict regulation around the use of chemicals in water cannon is a problem she would like addressed: "It does lead me to consider [this practice] as an experimental weapon. And populations should never be subjected to experiments. This is absolutely in violation of human rights law."

She stressed that any effect of a riot control measure should be temporary under international law, and that the symptoms described "are beyond what would be considered temporary and acceptable. So all of those cases should be investigated, including under the rubric of torture or other ill-treatment".

Georgia's authorities described our findings as "deeply frivolous" and "absurd".

It said that law enforcement had acted "within the bounds of the law and constitution" when responding to the "illegal actions of brutal criminals".

The protests on Tbilisi's Rustaveli Avenue have dwindled in their size since the government increased fines and jail terms, but not in their frequency.

Almost every night for the past year, demonstrators have called for the resignation of a government they accuse of rigging elections, siding with Russian interests, and passing increasingly draconian legislation against civil society.

The ruling Georgian Dream party denied the government or the party's honorary chairman, Bidzina Ivanishvili, are either pro-Russian or pursuing Russian interests. It told the BBC that legislative changes over the past year served the best interests of "public welfare".