Kremlin critic Alexei Navalny was fighting for his life in a Siberian hospital on Thursday after drinking tea that allies said they believe was laced with poison.

Hamad Medical Corporation (HMC) warned the public about the dangers of carbon monoxide poisoning from burning coal and firewood indoor for heating.

Police were called to the Prezzo restaurant in the evening in response to a "medical incident" involving a man and a woman, Wiltshire Police said in a statement.

Charlie Rowley's condition had improve overnight and was now "serious, but stable".

Russian President Vladimir Putin on Wednesday urged ‘common sense’ to prevail in the spy poisoning crisis as London and Moscow faced off in a tense meeting at the world's chemical weapons watchdog.

Russian television showed passengers disembarking from the plane while several buses waited to pick them up.

The move came after British Prime Minister Theresa May addressed fellow European Union leaders at a summit in Brussels to urge them to support Britain's assessment that Russia was to blame.

British Prime Minister Theresa May on Thursday warned EU leaders that Russia posed a threat to the whole bloc as she sought united backing against Moscow after the poisoning of an ex-spy in England.

A second police officer is being treated in hospital for suspected poisoning after a chemical attack on a former Russian double agent but the symptoms are minor, the Mail Online reported on Thursday.

Britain has said Russia used the Soviet-era nerve agent called Novichok to attack Sergei Skripal and his daughter Yulia in the first known offensive use of such a weapon on European soil since World War Two.

"Moscow does not accept unfounded accusations that are not based on evidence and a language of ultimatums," Kremlin spokesman Dmitry Peskov told reporters.

Sixteen Malaysian athletes have been hit by food poisoning at the Southeast Asian Games and one is in hospital, organisers said on Thursday in just the latest mishap to hit the competition.