Italian authorities last month seized hundreds of thousands of bottles of 2003 Brunello di Montalcino because of the suspected use by winemakers of grapes other than the Sangiovese, the only ingredient allowed in the premium Tuscan wine.
The allegations were first made in a news magazine report and the Agriculture Ministry has confirmed that hundreds of thousands of bottles of the wine have been seized by investigators.
A letter by the US Treasury's Alcohol and Tobacco Tax and Trade Bureau to the Italian embassy in Washington said US customs authorities had been told to "withhold the release of all shipments" of the wine from June 9 unless Italy or importers could demonstrate only Sangiovese grapes were used.