Russian billionaire Roman Abramovich suffered symptoms of suspected poisoning at talks in Kyiv earlier this month, sources close to him say.
The Chelsea FC owner – who has now recovered – reportedly suffered sore eyes and skin peeling.
Two Ukrainian peace negotiators were also said to have been affected.
The Wall Street Journal reported claims the alleged poisoning was orchestrated by hardliners in Russia who wanted to sabotage the talks.
The condition of Mr Abramovich and the Ukrainian negotiators, who include Ukrainian MP Rustem Umerov, has improved since the incident on 3 March, the paper quoted sources as saying.
A source close to Mr Abramovich told the BBC he had suffered symptoms of suspected poisoning. They said he had now recovered and was continuing with negotiations to try and end the war in Ukraine.
Mr Abramovich travelled between Moscow and Kyiv for several rounds of talks at the start of the month.
BBC