Update: Multiplayer maps Boardwalk, Gulch and Parish now available to download, according to CVG readers and Activision, although some users are still reporting getting XBL Marketplace error messages.
According to the online service's status page, "users will have issues accessing new MW3 content.
"We are aware of the problem and are working to resolve the issue," Microsoft added. "We apologize for any inconvenience this may cause and thank you for your patience."
Call of Duty's UK community manager also said on Twitter: "Things have been done a little differently for our ELITE drop in this instance. No idea on the length of the delay though."
CoD multiplayer fans who have been underwhelmed by MW3's Face Off and Spec Ops DLC should be looking forward to the final Elite offering, which consists of three new online maps: Gulch, Boardwalk and Parish.
They're scheduled to be made available to non-Elite subscribers on Xbox 360 tomorrow, September 6 as part of Content Collection 4, which also features Offshore and Decommission, maps Elite users received access to in July.