The site said the three men had only consumed water and chocolate – while another man who ate and drank the same things did not experience symptoms.

It continued: ‘The negotiation round on the afternoon of 3 March took place on Ukrainian territory, and lasted until about 10 pm.

‘Three members of the negotiating team retreated to an apartment in Kyiv later that night and felt initial symptoms – including eye and skin inflammation and piercing pain in the eyes – later that night.

‘The symptoms did not abate until the morning.’

President Zelensky’s spokesman said he had no information about any suspected poisoning, according to the WSJ.

It cited western experts suggesting it was difficult to confirm if the symptoms were caused by a chemical or biological agent or by an electromagnetic-radiation attack.

Bellingcat claimed the most likely cause of the symptoms was an ‘international poisoning with an undefined chemical weapon’, adding that ‘microwave irradiation’ was another possible theory.

It cited two chemical weapons experts and a doctor suggesting the symptoms were ‘most consistent with variants of porphyrin, organophosphates, or bicyclic substances’.

‘The experts said the dosage and type of toxin used was likely insufficient to cause life-threatening damage, and most likely was intended to scare the victims as opposed to cause permanent damage’, Bellingcat tweeted.

A person familiar with the matter told Reuters that Abramovich had not allowed it to stop him working.

The Kremlin has said the oligarch played an early role in peace talks between Russia and Ukraine but that the process was now in the hands of the two sides’ negotiating teams.