Russian opposition leader Alexei Navalny has been taken out of an induced coma after his suspected poisoning with the novichok nerve agent.
Mr Navalny, 44, is now said to be responding to speech at the Berlin hospital where he has been treated since last month.
Doctors added that despite the improvement in his condition, "long-term consequences of the serious poisoning can still not be ruled out".
A tweet from Mr Navalny's press secretary said he was going to be gradually disconnected from a ventilator.
It comes as the UK summoned Russia's ambassador and reprimanded his country for using a "banned chemical weapon".
Mr Navalny, a fierce critic of President Vladimir Putin, fell ill on a domestic flight to Moscow on 20 August, and had been in an induced coma since he was flown to hospital in the German capital two days later.
"The patient has been removed from his medically-induced coma and is being weaned off mechanical ventilation," the Charite hospital said.
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"He is responding to verbal stimuli. It remains too early to gauge the potential long-term effects of his severe poisoning."
It added the decision to publicly release details of his condition was made in consultation with his wife, Yulia Navalnaya.
Foreign Secretary Dominic Raab tweeted: "Today the UK summoned Russia's Ambassador to the UK to register deep concern about the poisoning of Alexey Navalny.
"It's completely unacceptable that a banned chemical weapon has been used and Russia must hold a full, transparent investigation.
"Relieved to hear that Alexei Navalny has been taken out of the medically induced coma. I hope his condition continues to improve."
Last week, German officials said tests showed Mr Navalny had been poisoned with novichok – the same nerve agent used on the Skripals in Salisbury which was developed by the Soviet Union in the 1970s and 1980s.
German Chancellor Angela Merkel said the use of novichok showed the "dangerous" attack on Mr Navalny was attempted murder and the aim was to silence him.
She said she has contacted the Organisation for the Prohibition of Chemical Weapons – and expects Russia to carry out an investigation into who was responsible as there are "very serious questions that only the Russian government can answer and must answer".
Russia has denied the Kremlin was involved in poisoning Mr Navalny and accused Germany of failing to provide evidence about the poisoning that it requested in late August.
On Sunday, the German government threatened to rethink its underwater pipeline with Russia, unless the country began cooperating with its investigation into the incident.
The UK has condemned the suspected attack on Mr Navalny as "utterly deplorable" and is working with Germany to "ensure Russia was held accountable for its international obligations".