It has been suggested that the problems may be linked to the release of new Halo 4 DLC on Monday.
"There are a few hundred players who have been unable to connect to our servers since yesterday," acknowledged a member of the Halo Waypoint forum team. "We're investigating now and hope to have an update soon."
Another moderator said more recently: "The number of players in the fallback hoppers has dropped, however there are still people experiencing problems accessing the main servers. We'll continue to investigate and push out fixes until these issues are resolved."
An entry on the Halo Waypoint Twitter account also says: "We're actively investigating the server issues. Apologies for the inconvenience."
343 has requested that players experiencing connection problems share their Gamertag and location on the Halo Waypoint forums.