Fortnite has gone down while new fixes are applied to the game.
It will be unplayable for a short time while developers do "maintenance".
No major updates are expected for players when the game goes back online, since the update was announced only as a "maintenance patch".
Such scheduled updates usually take just a couple of hours before allowing players to get back online.
The outage began around 7am UK time, and developers Epic Games gave no indication of when it might be over.
Epic also did not release patch notes for the update, or indicate which bugs or problems it will fix.
Scheduled updates of this kind are usually timed to minimise disruption to players. While the update came during the early morning in the UK, that meant most players in the US and elsewhere were more likely to be offline.
Huge numbers of people are thought to be playing the game as they are forced into lockdown in many countries across the world. Epic does not give live indications of how many people are playing its games but the number of people streaming it on Twitch, for instance, has increased considerably in recent weeks.
In March, as the country was instructed to stay inside, Italian internet companies said they were seeing a surge in internet traffic coming from games like Fortnite and Call of Duty. "We reported an increase of more than 70% of internet traffic over our landline network, with a big contribution from online gaming such as Fortnite," Telecom Italia chief executive Luigi Gubitosi said on a conference call reported by Bloomberg last month.
