"The poisoning was carried out so professionally that no other conclusion is possible," the activist said.

He fell ill after a court hearing, losing his sight and ability to speak.

He was flown to Berlin for treatment, but discharged from hospital on Wednesday.

In an interview with the BBC, Mr Verzilov said he believed he was most likely attacked either by Russia's FSB domestic security agency or GRU military intelligence.

"The actions we do are as loud as anything happening in Russia, so for them it's quite a big deal and that's just the price you have to pay if you want Russia to change," he said.

The Berlin Charité hospital where he was treated issued a press release on 18 September stating Mr Verzilov was admitted with "symptoms of poisoning".

Staff now believe that, although there are no traces of poison in his system, there is no other explanation for his condition.

Pyotr Verzilov arrived in Berlin on a special medical transport plane

The activist told Germany's Bild newspaper they could have tried a "new cocktail of poisons" on him, as his symptoms were more immediate "than others".

He says they may have tried to poison him for two reasons.

The first could be his part in a pitch invasion during the 2018 World Cup final, for which he and three other Pussy Riot members were briefly jailed.

The second, for investigating "three Russian journalists who were murdered in Africa".

Journalists Orkhan Dzhemal, Alexander Rastorguyev and Kirill Radchenko all died in the Central African Republic in an ambush over the summer.

From L-R: Kirill Radchenko, Alexander Rastorguyev and Orkhan Dzhemal

Their mission was to look into reports that fighters from a Russian private military company (PMC), the Wagner Group, were operating there.

Mr Verzilov thinks his investigation into their deaths "is a more likely reason than the World Cup initiative."

One day before his suspected poisoning, Mr Verzilov says he received news about the CAR deaths.

"We cannot comment on this publicly yet, but there is some news," he told Bild.

Mr Verzilov said he had every intention of returning to Russia once he had recovered.

"I assume that those who once poisoned him will try again. That's why I do not want him to go back," his girlfriend Veronika Nikulshina told the paper.

"But you cannot stop Pyotr."