Shipping food rather than cash to disaster-hit poor countries cuts the benefit of such aid by a third, according to a new study that raises a fraught issue in world trade negotiations.

The study, published by the Organisation for Economic Co-operation and Development, the Paris-based intergovernmental think-tank, says that “tied” food aid, shipped as surpluses from the donor country's farmers, often arrives too late and is more expensive than buying it locally.