Nottingham Forest and Olympiakos owner Evangelos Marinakis has tested positive for coronavirus.
Marinakis, 52, was at the City Ground for Forest's Championship fixture against Millwall on Friday.
Olympiakos are due to host Wolves in the Europa League on Thursday, although the BBC understands Wolves have asked for the game to be postponed.
"Mr Marinakis was diagnosed after showing the first symptoms on his return to Greece," said Forest.
"During his stay in Nottingham, he did not show any symptoms of the virus.
"The club are seeking advice from medical professionals and the relevant governing bodies to ensure the correct measures are taken."
Marinakis had earlier revealed he had been diagnosed with coronavirus on social media platform Instagram.
"The recent virus has 'visited' me and I felt obliged to let the public know," he said. "I feel good as I take all the necessary measures and I discipline to the doctor's instructions."
Millwall, who won Friday's game in Nottingham 3-0, have said that "senior club representatives who came into contact with Mr Marinakis have begun a period of self isolation".
A club spokesman added: "This is purely a precautionary measure and no individual has displayed any symptoms. The club will continue to follow all necessary guidance from the appropriate bodies."
The Lions are due to host Derby County at The Den on Saturday.
It was confirmed on Monday that Wolves' last-16 first-leg match at Georgios Karaiskakis Stadium was to be played behind closed doors because of the coronavirus outbreak.
The Premier League club had sold 1,000 tickets for the fixture, with full ticket refunds currently being processed.
The Greek government announced on Sunday that all professional sports events for the next two weeks will be played without spectators, although Culture Secretary Oliver Dowden has said that events in Britain are unlikely to be affected by coronavirus in the immediate future.
Forest, who are fifth in the Championship table and scheduled to play Sheffield Wednesday in their next league game on Saturday, have been in contact with the English Football League.
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