Arsenal have announced their head coach Mikel Arteta has contracted the coronavirus.
The whole first-team squad will now be placed in self-isolation.
A club statement read: "Our London Colney training centre has been closed after head coach Mikel Arteta received a positive Covid-19 result this evening.
"Arsenal personnel who had recent close contact with Mikel will now self-isolate in line with Government health guidelines. We expect this to be a significant number of people from Colney, including the full first-team squad and coaching staff."
Arsenal 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."
Arsenal are now trying to trace any other people who may have had contact with Arteta.
The statement added it was "clear we will not be able to play some fixtures on their currently scheduled dates".
The Gunners were due to travel to Brighton on Saturday but the game has been postponed with both clubs confirming that the game is off.