Amazon is banning its third-party merchants from using FedEx’s ground service to deliver to Prime members, suggesting that it thinks the service is too slow to get packages to their destinations in time for Christmas.

The temporary ban will block those companies from using FedEx Ground service, although they can still use pricier FedEx Express shipping for Prime shipments.

More than half of the items sold on Amazon.com come from third-party sellers, who post their goods for sale on Amazon’s online marketplace.