EA working to resolve an issue.
FIFA 13 Ultimate Team has been temporarily disabled in order for EA to get to the bottom of recent issues with the service, explained the publisher on the EA forums.
"While we work to resolve an issue we've temporarily disabled FIFA 13: Ultimate Team. This is to ensure that an optimal experience can be sustained in the future," reads the post. "FUT13 online gameplay, trading, store purchasing, the Web App, and the mobile companion app will be unavailable during this time.
"We'll keep you updated as soon as we have additional information throughout the coming days. Other FIFA 13 online game modes will not be affected."
As a result, Team of the Week has been delayed until the problems can be addressed.
Since launch FIFA 13 Ultimate team has been plagued with problems, although EA's efforts now appear to be focussed on buying and selling players – EA took the auction system offline yesterday.