Mikel Arteta has tested positive for coronavirus and the Arsenal squad is expected to self-isolate, the Gunners have confirmed.
The club said in a statement released on Thursday that its London Colney training centre has been closed as a result of the diagnosis.
Arsenal confirmed staff and players who had "recent close contact" with Arteta would go into self-isolation, which includes the entirety of the first team.
"Arsenal personnel who had recent close contact with Mikel will now self-isolate in line with government health guidelines," a statement from the Gunners read.
"We expect this to be a significant number of people from Colney, including the full first-team squad and coaching staff, as well as a smaller number of people from our Hale End Academy which we have also temporarily closed as a precaution.
"We expect those who did not have close contact with Mikel to return to work in the coming days. In the meantime our Colney and Hale End training centres will undergo a deep clean and our other club sites are operating as normal."
Arteta said: "This is really disappointing but I took the test after feeling poorly. I will be at work as soon as I'm allowed."
Arsenal's announcement came shortly after the Premier League confirmed this weekend's fixtures, including the Gunners' trip to Brighton and Hove Albion, will go ahead as planned.
But following Arteta's positive test it seems likely Arsenal's schedule will face further disruption after Wednesday's game against Manchester City was called off.
That match was suspended after news broke that Olympiacos owner Evangelos Marinakis had contracted coronavirus.
Olympiacos knocked Arsenal out of the Europa League in February and Marinakis was in attendance for the second leg at Emirates Stadium.
Arsenal indicated it expect further fixture postponements amid the coronavirus pandemic.
"It is clear we will not be able to play some fixtures on their currently scheduled dates," the club said.
The Premier League's announcement that games would continue came after British prime minister Boris Johnson indicated the government was considering postponing sporting events.
However, in response to the news that Arteta had contracted COVID-19, the Premier League called an emergency club meeting for Friday.
Arsenal's managing director Vinai Venkatesham said: "The health of our people and the wider public is our priority and that is where our focus is. Our thoughts are with Mikel who is disappointed but in good spirits.
"We are in active dialogue with all the relevant people to manage this situation appropriately, and we look forward to getting back to training and playing as soon as medical advice allows."
The club's head of football Raul Sanllehi added: "Mikel and the full first-team squad, players and staff, will be fully supported, and we look forward to getting back to training and playing as soon as medical advice allows.
"Obviously Mikel's full recovery is the priority now for us all."
Hudson Odoi tested positive too, confirms Chelsea
Chelsea forward Callum Hudson-Odoi has tested positive for coronavirus, the Premier League club confirmed.
Hudson-Odoi, 19, became the first Premier League player to test positive for coronavirus and the entire Chelsea squad, coaching staff and several backroom staff will self-isolate.
The men's team building will be closed, but the rest of the club's training facility and Stamford Bridge are operating as normal.
"Callum displayed symptoms similar to a mild cold on Monday morning and has not been at the training ground since then as a precaution," a statement read.
"However, his test came in positive this evening and he will undergo a period of self-isolation. Despite testing positive for the virus, Callum is doing well and looking forward to returning to the training ground as soon as it is possible."
It added: "We wish Callum a speedy recovery and look forward to welcoming him back to the club soon."