Tests indicate that Alexei Navalny was the victim of a poisoning and he is being treated with atropine, the same antidote used after the 2018 nerve agent attack in Salisbury, the German clinic where the Kremlin critic is a patient said on Monday. While Berlin’s Charité hospital did not identify the specific poison responsible for Navalny’s sudden illness on an internal Russian flight last Thursday, the substance was part of a group that affects the central nervous system, and includes nerve agents and pesticides, as well as some drugs. The statement was the first medical corroboration of a poisoning attack on Navalny and marked him as likely the latest Kremlin opponent to face an attempt on his life. In a statement, the hospital said Navalny had fallen ill because of contamination from a cholinesterase inhibitor, adding that the specific substance was not known and analysis was ongoing. Cholinesterase inhibitors block an enzyme that is necessary for the proper function of the nervous system. According to the clinic, Navalny is in serious condition but “there is currently no acute danger to his life”. It said in a statement released online: “Alexei Navalny’s prognosis remains unclear; the possibility of long-term effects, particularly those affecting the nervous system, cannot be excluded.” Hamish de Bretton-Gordon, a chemical and biological weapons expert, said the presence of high levels of cholinesterase suggested Navalny could have been poisoned with a nerve agent. “The levels show how much you have been exposed. It’s just a touch suspicious that someone vehemently opposed to the Russian state should fall ill in this way.” Other high-profile attacks against Kremlin opponents and dissidents have included the 2018 poisoning of Sergei Skripal with the nerve agent novichok, the 2015 shooting death of the opposition politician Boris Nemtsov, and the 2006 poisoning with a radioactive isotope of Alexander Litvinenko, an FSB defector. Following the statement, Leonid Volkov, a Navalny ally, quickly drew comparisons with the Salisbury poisoning: “The world’s most famous cholinesterase inhibitor is called novichok.” Navalny was being treated with atropine, an antidote used to treat the victims of poisoning by nerve agents and pesticides, the clinic said. Doctors in Salisbury had used the same antidote to save the lives of Skripal and his daughter, Yulia, after the 2018 attack with the nerve agent novichok. A hospital in the Russian city of Omsk had previously denied that Navalny had been poisoned. On Monday, the hospital’s deputy chief doctor insisted that two laboratories had found no toxins or poisoning products in Navalny’s tests. Supporters said doctors there were under government pressure to cover up any evidence of an attack against the opposition critic. “We were sure that Alexei had been poisoned … despite all the statements from Russian propaganda and doctors,” said Kira Yarmysh, a spokeswoman for Navalny, in a statement sent to the Guardian. “Now the poisoning has been confirmed. It is not a hypothesis any more, it is a medical fact.” Navalny was evacuated to Germany on Saturday after a direct plea from his wife to Vladimir Putin, the Russian president, to allow his transfer abroad, as well as public concerns expressed by the leaders of Germany, France and Finland. He had fallen ill during a flight in Russia last week. International leaders have voiced concern over the Russian opposition critic’s health and confirmation of his poisoning will likely prompt a wave of condemnation of the Kremlin. The German chancellor, Angela Merkel, called in a statement on Monday evening for Russia to investigate the poisoning and hold the perpetrators to account. “Those responsible must be found and brought to justice,” she said. The US ambassador to Moscow, John Sullivan, told journalists earlier on Monday that confirmation of Navalny’s poisoning would “represent a crucial moment for Russia”. The deputy secretary of state, Stephen Biegun, was due to arrive in Russia on Tuesday as part of a tour through the region that would also include discussions of the Belarus crisis. “The Russian people deserve to see that anybody who would have been involved in a matter like that is held accountable,” he said. “If Navalny had been poisoned, this is very significant for the United States.” Supporters have said that they believe the Kremlin is behind the attack. Lyubov Sobol, a Navalny ally whose husband was stabbed with a syringe in a 2016 attack that sent him into convulsions, told the Guardian that similar attacks were the “hallmark of the Kremlin”. She and others said that they believed that such an attack could only be authorised by a senior Russian official with Putin’s knowledge. Security has been increased at the Charité hospital complex. The German government said earlier on Monday that Navalny required police protection because of a “certain likelihood” he was poisoned, a spokesman has said. The opposition leader was transported in an isolation pod often used for coronavirus patients owing to concerns that whatever caused his sudden illness could place medical staff or the air crew at risk. Supporters suspect that he may have been targeted with poisoned tea before boarding a flight last week to Moscow. He lost consciousness soon after takeoff. On Monday, Yarmysh, the aide to Navalny, said that three days after filing a complaint with Tomsk police, a criminal case still had not been opened into the suspected attack. Navalny’s supporters also claimed he was under surveillance during his trip to Siberia. A newspaper article citing police sources detailed extensive government surveillance of Navalny before he fell ill. According to the article, police identified the apartment where Navalny was staying by tracking a sushi delivery to an associate, collected his receipts from a local store, and even followed him during a short trip out of town for an evening swim in the Tom River. On Monday, the Kremlin spokesman, Dmitry Peskov, would neither confirm or deny that Navalny was being watched by the security services. “All I can do in this case is refer your question to the security services,” he told journalists.