The US states claim that the heat treatment in ISPM 15 will not prevent pests from entering the US, and that the other treatment permitted – methyl bromide – is an ozone-damaging gas.

But the states’ argument that plastic or plywood pallets are pest-free and better for the country has been challenged by the NAPD. It said the claim ignores the fact that the USA Plant Protection Organisation took part in the creation of ISPM 15, and that they and Canada and Mexico have already implemented it.

However, the NAPD says a switch to plywood or plastic pallets is not the answer as both are expensive and plastic pallets use oil and fossil fuels while timber pallets come from a sustainable resource.

Paul Tait, NAPD secretary, said: “Our members repair, recycle and treat pallets to the highest international phytosanitary standards.

“The promotion by certain US states of plastic pallets by attempting to eliminate sawn timber is unrealistic in a country that produces some 200 million pallets of its own each year.

“Cutting down trees from managed forests and processing wood has a very low environmental impact and we strongly support the US wood pallet trade and its trade body the NWPCA in their efforts to fend off this legal challenge.”