The transport giant is experiencing “considerable problems” in customs formalities in the movement of goods between the UK and EU, it said in a statement sent to AFP on Friday.
Only “around 10 percent” of deliveries are accompanied by the complete and correct paperwork, it added.
DB Schenker, owned by Germany's state-owned rail giant Deutsche Bahn, said it has staff specially trained to deal with Brexit issues but “every shipment that is not properly documented delays the delivery of entire loads”.
Shipments that are not fully declared cannot be delivered, it said.
The company expects a further increase in the volume of shipments in January, but said it “will only be able to process these quickly if the proportion of deliveries with complete paperwork increases significantly.”
“Both the sender and the recipient have a duty to provide compliant documents for this purpose.”
It said deliveries already posted will continue to be transported as quickly as possible.
The post-Brexit trade agreement concluded on Christmas Eve ended the free movement of goods between the EU and Britain.
The French parcel delivery service DPD said earlier in January that it was suspending deliveries from Britain to EU countries for a few days.
It said it needed time to adjust to the “new, more complex procedures” for passing through customs because of Brexit.
DB Schenker handles around half a million shipments from continental Europe to the UK every year.