The Kremlin has rejected accusations of involvement in an alleged attack on Russian opposition leader Alexei Navalny who is in a coma in a German hospital, a day after doctors said tests indicated that he was poisoned.

The politician’s allies say the Kremlin is behind the illness of its most prominent critic, with some demanding an investigation into whether Russian President Vladimir Putin was involved.

“These accusations absolutely cannot be true and are rather an empty noise,” Kremlin spokesman Dmitry Peskov told reporters. “We do not intend to take it seriously.”

Kremlin critics are demanding a probe into whether Vladimir Putin was involved in Mr Navalny's illness
Kremlin critics are demanding a probe into whether Vladimir Putin was involved in Mr Navalny’s illness (Alexei Nikolsky, Sputnik, Kremlin Pool Photo via AP)

Mr Peskov saw no grounds for launching a criminal investigation into Mr Navalny’s condition, saying that it could have been triggered by a variety of causes, and determining one should come first.

“If a substance (that caused the condition) is found, and if it is determined that it is poisoning, then there will be a reason for an investigation,” Mr Peskov said.

Mr Navalny, a politician and corruption investigator who is one of Mr Putin’s fiercest critics, fell ill on a flight back to Moscow from Siberia on Thursday and was taken to a hospital in the city of Omsk after the plane made an emergency landing.

Over the weekend he was transferred to the Charite hospital in Berlin, where doctors said they have found indications of “cholinesterase inhibitors” in his system.

Cholinesterase inhibitors act by blocking the breakdown of a key chemical in the body, acetycholine, that transmits signals between nerve cells. Mr Navalny is being treated with the antidote atropine.

Mr Navalny’s wife, Yulia Navalnaya, has been visiting her husband every day and made no comment to reporters as she arrived on Tuesday.

Mr Navalny's wife, Julia, arrives at the Berlin Charite hospital
Mr Navalny’s wife, Julia, arrives at the Berlin Charite hospital (Christoph Soeder/dpa via AP)

Chancellor Angela Merkel personally offered Germany’s help in treating Mr Navalny and, along with other European officials, has called for a full Russian investigation.

Charite said Mr Navalny had undergone extensive examination by a team of physicians and that “clinical findings indicate poisoning with a substance from the group of cholinesterase inhibitors”.

Cholinesterase inhibitors are a broad range of substances that are found in several drugs, but also pesticides and nerve agents. Charite said the specific substance to which Mr Navalny was exposed is not yet known but that a further series of comprehensive tests had been started.

Mr Navalny’s spokeswoman, Kira Yarmysh, said the Russian government’s reluctance to launch an investigation was expected.

“It was obvious that the crime would not be properly investigated and a culprit found. However, we all know perfectly well who that is,” Ms Yarmysh tweeted.

Experts have warned that it is far too early to draw any conclusions about how the agent may have entered Mr Navalny’s system, but note that Novichok, the Soviet-era nerve agent used to poison former Russian spy Sergei Skripal and his daughter in Salisbury, was a cholinesterase inhibitor.