Mr Navalny, a 44-year-old lawyer and anti-corruption campaigner who is among President Vladimir Putin's fiercest critics, was hospitalised in the city of Omsk after he lost consciousness while on a flight and his plane made an emergency landing.
"Alexei has toxic poisoning," Ms Yarmysh wrote on Twitter, describing how he was taken ill during the flight from the city of Tomsk to Moscow. "I hope that he can recover and... he can receive from us all the help and medical support needed," Ms Merkel said in a joint news conference with French President Emmanuel Macron. Mr Navalny's team had said earlier that the hospital in Omsk was ill-equipped and his doctor Anastasia Vasilyeva said she had asked for the Kremlin's help to transfer him to a European clinic.
"We can't rule out that he drank or took something himself yesterday," the source said, a claim Ms Yarmysh dismissed as "complete rubbish". Amnesty International urged Russia to hold a "prompt and independent investigation" into the incident while European Union foreign affairs chief Josep Borrell said "those responsible must be held to account" if the suspected poisoning was confirmed.