New scientific studies suggest that an artificial hormone used in the US to make cattle produce more milk could raise the risk of breast and prostate cancer in humans.
Yet the American government is now pushing Europe to end a moratorium on the use of animal growth hormones. The moratorium was imposed in 1989 on the grounds that European farmers were already producing too much milk and beef, and there was no need for greater production. It was extended in 1996 and 1997.
But the US says now that the ban infringes free trade rules - and, in particular, restrains the ability of Monsanto, which has patented a genetically engineered version of the milk-producing hormone, to sell its product in Europe.
The row carries echoes of the ongoing arguments over imports of bananas from Caribbean countries to the EU, which, according to the US, breach trade rules. Monsanto's presence also recalls the lack of labelling on genetically-modified soya beans grown in the US but shipped to Europe.
But since the European ban on bovine somatotropin (BST) came into force in 1997, new scientific evidence has found that giving cattle excess levels of the hormone - a naturally-occurring substance - can cause a fivefold rise in the levels of a protein called IGF-1 (immune growth factor) in the milk.
Studies published last year in Science and The Lancet, showed that heightened levels of IGF-1 in humans carry an increased risk of prostate cancer in men, and breast cancer in women.
Senior European vets are understood to have submitted a report to the European Commission calling for the ban on BST to continue. That could trigger a renewed trade row with the US.
BST, produced by genetically engineered bacteria carrying the cattle DNA for the hormone, is marketed by Monsanto under the name Posilac. It was patented, and won approval for use from the US Food and Drug Administration in 1993.
But the quality of the FDA's investigation has now been called into doubt. Critics say that rather than carrying out its own studies on the hormone, it relied on studies summarised and passed to it by Monsanto. A search by The Independent of the FDA's literature on BST does not show any investigation of IGF-1 levels in milk produced by injected cattle.
A spokesman for Monsanto dismissed the concerns and insisted the product was safe.