At first, carriers turned to barges as a form of "buffer storage" to avoid sending empties further upstream, the vessels holding around 400 TEU close to ocean terminals.

But empties continued to scatter throughout Europe, which led carriers leasing barges, believing they could be more efficient retrieving boxes themselves.

Two of the big five container lines had already begun operations, with three trips over the past 10 days moving between 4,000 and 6,000 TEU of empties.

"One out of every two containers moving from Europe to Asia is empty. Many of these in Belgium and the Netherlands are held at inland sites and getting them back to ocean terminals has proved to be a nightmare," a source told Loadstar.

"Ocean carriers are not geared to operating inland navigation, but this emergency usage is making them consider it. I know at least one of the carriers to be in negotiations with barge owners," the source said.

Hapag-Lloyd is one carrier not using leased barges "at the moment", said a spokesman, adding that one carrier "desperately" seeking space for 500 TEU of empties was only offered 15 TEU by a barge operator prioritising pre-existing clients.

"Vendors cannot guarantee space; they cannot guarantee they will get it there on time, as the terminal will only handle the barge when it has space," he said. "This means the shipping line is reliant on an unreliable process and, even if it gets the space on the barge, that barge may not be handled in time for the ocean ship's departure.