The popularity of 40 ft high cubes (HCs) is causing stowage problems for shipping carriers, according to Drewry.

In its latest Container Insight Weekly, the firm says 9 ft 6 in containers now need to be stored below deck, which risks reducing available cargo space.

Likewise, it warns it is difficult to measure in terms of twenty-foot equivalent units (teu) - a popular measurement in the sector - as 40 ft HCs count as two teu, the same as two 20 ft boxes or one traditional 40 ft container. As a result, teu figures are often being underestimated by ships using HCs.

A spokesperson said: "The difference may not seem much, but it makes a big difference to economists trying to make sense of the changes between GDP growth and cargo growth. It is also cumulative, so gets bigger over longer periods of time."

Yet Drewry also understands the popularity of 40 ft HCs, as they are around 13 per cent bigger than traditional 40 ft boxes and allow ships to typically load the extra freight at less cost. It is also suitable for hauling, as light cargo is usually only charged for each container.