WASHINGTON -- A U.S. court that only days ago upheld a ruling that allowed country of origin meat labelling to continue now says it will re-hear the case.

The U.S. Court of Appeals for the District of Columbia Circuit said Friday it was vacating its ruling on March 28 that denied a request for an injunction against the meat labelling rules.

The court says it will hear the case again on May 19 in front of all of the court's judges.

The injunction is being sought by groups such as the American Meat Institute and the Canadian Cattlemen's Association, which have launched a lawsuit against the U.S. Department of Agriculture.

The groups argue the rules go beyond what lawmakers intended and violate First Amendment rights to freedom of speech.

Country of origin labelling (COOL) rules took effect last year and require packaged steaks, ribs and other cuts of meat to include country of origin labels.

The court has asked both sides to file submissions addressing the First Amendment issue.

"It's a very unusual development," Janet Riley of the American Meat Institute said on Saturday. "Our attorney says it happens only once every few years."

The industry groups argue the rules violate the U.S. Constitution because they force meat producers to provide information about their products, and that the information is of no real value to the consumer.

In the March 28 ruling, Judge Stephen F. Williams wrote the labelling is a "minimal" intrusion on the meat industry's First Amendment rights, noting it allowed consumers to "apply patriotic or protectionist criteria in the choice of meat." He said it also allowed consumers to make choices based on their belief that U.S. food safety is superior.

But the ruling contained a footnote that stated "reasonable judges" could interpret rulings that were cited in cases related the First Amendment issue differently and suggested the full court hear the case to resolve the issue.

Riley said that when the groups seeking the injunction saw the footnote, they weren't sure what to do next. Was it up to them to ask for the court to hear the case again, they wondered?

In the end, the court initiated the action itself.

"A vote was taken and a majority of the judges eligible to participate voted to re-hear the case, an order by the court issued on Friday stated.

The Canadian Cattlemen's Association said it was pleased about the decision to re-hear the case.

"(The) development is another step in the ongoing battle over COOL," the association said in a statement.

"The cost of COOL to Canadian producers and industry is unacceptable and CCA will continue until a viable remedy is reached."

The Canadian government wants the labelling rules eased, saying they complicate trade and are costing Canadian producers a billion dollars a year.