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.
The Russian billionaire reportedly met President Volodymyr Zelensky during the trip, but the Ukrainian leader was not affected and his spokesman had no information about the incident.
The investigative journalism group Bellingcat said Mr Abramovich and the negotiators suffered symptoms “consistent with poisoning with chemical weapons”.
The symptoms included “eye and skin inflammation and piercing pain in the eyes”, Bellingcat reported.
The businessman has since been seen in public, being pictured at Israel’s Tel Aviv airport on 14 March.
A spokesman for Mr Abramovich previously said he was playing a “limited” role in trying to find a “peaceful resolution” between the two countries.
Mr Abramovich was sanctioned by the UK government earlier this month over his alleged links to Russia’s President Putin.
This breaking news story is being updated and more details will be published shortly. Please refresh the page for the fullest version.
You can receive Breaking News on a smartphone or tablet via the BBC News App. You can also follow @BBCBreaking on Twitter to get the latest alerts.