A statement from two government ministries said Japan informed the US Embassy of the findings and that shipments had been temporarily halted from the California plant involved.

Mr David Marks, a spokesman for the US Embassy in Tokyo, confirmed that US officials were aware of the incident and said a full investigation would be conducted 'to find out how it happened.'

He added, however, that there was nothing inherently wrong with the product in question.

'While we recognise that this is a product that doesn't meet Japanese standards, the product is perfectly safe, and the international animal health organisation has determined that all meat, all cuts, all ages of American beef are safe,' Mr Marks said.

The spinal column was discovered on Monday at a Japanese meat-processing factory during an inspection, said the statement from the Ministry of Agriculture, Forestry and Fisheries and the Ministry of Health, Labour and Welfare.

It was found in one of 700 boxes shipped from National Beef California L.P. and imported by trading house Itochu, the statement said. Itochu had sold the meat to Yoshinoya Holdings, operator of a fast-food chain that uses US beef for its popular beef bowl dish.

Calls to parent company National Beef Packing in Kansas City were not immediately returned.

The box containing the spinal column was mislabeled and contained a cut of beef that Yoshinoya had not ordered, said a Ministry of Health spokesman who asked not to be named because he was not authorised to speak to media. The other 699 boxes remain in a warehouse while Japanese officials await a US response.

Itochu said in a statement that no products from the 700-box lot, which had been in storage since arriving in Japan in August, have been released into the market.

Yoshinoya will continue selling beef bowls as usual, Kyodo News agency reported.

Major supermarket chain Daiei, however, said it will suspend sales of beef from the California plant for an unspecified period, according to Kyodo.

Japan imposed a ban on US beef imports in December 2003 after the first case of mad cow disease was found in the United States.

The ban was lifted in late 2005, only to be imposed again in January 2006 after inspectors found prohibited animal parts in a veal shipment from New York.

The agreement between the two countries states that meat shipped to Japan comes only from cattle age 20 months and younger, which are thought to pose less of a risk of the disease. US exporters must also remove spinal columns, brain tissue and other materials from shipments bound for Japan.

US beef imports resumed in July 2006, but sales are a fraction of what they used to be.

Mad cow disease, formally known as bovine spongiform encephalopathy, is a degenerative nerve disease in cattle. In humans, eating meat products contaminated with the illness is linked to variant Creutzfeldt-Jakob disease, a rare and fatal malady. -- AP