West Ham United manager David Moyes and two of the club's players have tested positive for coronavirus.
The club were informed of the positive tests while preparing for Tuesday's home Carabao Cup tie against Hull City - which the Hammers won 5-1.
Moyes, 57, defender Issa Diop, 23, and 24-year-old midfielder Josh Cullen will now be required to self-isolate.
"The manager and both players immediately left the stadium and have returned home," said a club statement.
West Ham said the three men were all asymptomatic and assistant manager Alan Irvine took charge of the Hull match at London Stadium, which both Diop and Cullen were due to start.
Irvine said: "At 18:20 we suddenly got some information from our doctor and head physio to say there were three positive tests, the manager and the two players.
"We'd had contact insofar as we were in the same area of the stadium as David.
We'd been at the stadium for about two hours before the match started.
"We were shocked by the news.
We've had so many tests and not had any positive.
But we've been very particular about protocol right from the start - the medical team are very aware of what has been happening and we've followed all the protocol, and we'll be following protocol now.
" Hull City said they consulted with the English Football League about whether the game should go ahead and the League One club said they felt "comfortable" after being "satisfied" with how West Ham followed guidelines.
Tigers manager Grant McCann said his team found out about the positive tests "just before the teamsheets" were submitted and that his players and staff "did not come into contact with Moyes or the others".
Public Health England guidelines state those with Covid-19 should self-isolate for at least 10 days, meaning Moyes, Diop and Cullen will not be able to attend training or the league game against Wolverhampton Wanderers on 27 September.
BBC Radio 5 Live's John Southall said the trio travelled to the ground with the squad in two or three separate coaches.
"They say they all social distanced, the squad, from when they met up and throughout the journey," he said.
"As soon as they arrived, the results from the tests done yesterday came through and the two players and the manager were isolated immediately in separate rooms and left the ground very quickly, I'm told.
"West Ham say they are satisfied nobody else in the squad needs to self-isolate and that is why the game went ahead.
" The news comes after Leyton Orient v Tottenham Hotspur in the EFL Cup third round on Tuesday was called off because a number of the home players had contracted coronavirus.
It came on the day the government announced a raft of new coronavirus restrictions for England which could last for up to six months.
As part of the stricter rules, Prime Minister Boris Johnson has confirmed plans for fans to return to watch live sport events in England from 1 October will not go ahead.
Sports bodies have been told not to expect spectators at stadiums and venues until the end of March at the earliest.