As reported by DownDetector, there has been a huge surge in the number of cases of players being unable to connect to the game's servers.
Call of Duty Servers Down
The DownDetector outage map shows the main areas as the UK and Europe, with some areas elsewhere affected.
Activision has now confirmed that they are investigating issues, with the publisher noting the following on their support page:
"We're currently experiencing connectivity issues. This incident is under investigation".
While DownDetector doesn't specify which CoD title is having issues, Activision has confirmed that it is 2019's Modern Warfare.
We'll do our best to keep you updated.