Slack's own service status page has acknowledged issues, saying that engineers are "working to restore functionality".
Five issues are raised, including trouble loading or using Slack, messaging, connectivity, Apps/Integrations/APIs and workflows.
More than 600 users reported that Slack is not working on Down Detector's UK site.
Hundreds of users have reported issues accessing the service, which is used by businesses across the world.
POPULAR messaging platform Slack has been hit by a mysterious outage.