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Chelsea Football Club owner Roman Abramovich was among a group who suffered symptoms consistent with poisoning after attending peace talks in Ukraine.
The Russian oligarch, who is involved in negotiations between Kyiv and Moscow, is understood – along with at least two Ukrainian negotiators – to have developed red eyes, constant and painful tears and peeling skin on their faces and hands since the meeting at the beginning of this month, The Wall Street Journal reported.
Sources told the PA news agency that Mr Abramovich had since recovered and was continuing to try to help with the negotiations.
It is understood the oligarch had been involved in talks about securing humanitarian corridors to allow Ukrainians to leave, and bringing other countries to the negotiating table.
The WSJ said the suspected attack had been orchestrated by hardliners in Russia who wanted to sabotage the talks.
The newspaper said Mr Abramovich met President Volodymyr Zelenskyy on the trip, but the Ukrainian leader was unaffected and his spokesman had no information about the alleged incident.
A spokesman for the businessman previously said he was playing only a "limited" role in trying to find a "peaceful resolution" between the two countries.
Mr Abramovich was hit by UK government sanctions this month over his connections with Russian President Vladimir Putin.
The government described him as "a prominent Russian businessman and pro-Kremlin oligarch" who is "associated with a person who is or has been involved in destabilising Ukraine and undermining and threatening the territorial integrity, sovereignty and independence of Ukraine", meaning Mr Putin.
Ukrainians attempt to stop Abramovich's yacht docking in Turkey
The government said Mr Abramovich had received financial benefits from the Kremlin, including tax breaks for his companies, the buying and selling of shares from and to the state at favourable rates, and contacts in the run-up to the 2018 World Cup in Russia.
"He is one of the few oligarchs from the 1990s to maintain prominence under Putin," a statement said.
Mr Abramovich has denied any association with the Russian regime.
