BRUSSELS - The European Union's transfer of airline passenger data to the United States -- part of U.S. efforts to fight terrorism -- should be declared illegal, an adviser to the European Union's highest court said on Tuesday.

Since May, 2004, the EU has shared with U.S. authorities 34 categories of information on airline passengers flying to U.S. destinations, including name, address, all forms of payment information and contact phone numbers.

The agreement sprang from one of the anti-terrorism laws passed by U.S. Congress in response to September 11, 2001, attacks using hijacked aircraft.

A court statement said: "Neither the (European Union) Council decision approving the agreement nor the (European) Commission decision holding that information be sufficiently protected by the United States have an adequate legal basis."

If the European Court of Justice accepts the advice of its adviser the data-sharing system will be made illegal.The Luxembourg-based court will likely rule next year. It follows the lead of its advisers in most cases.

The data case began last year when the European Parliament sued the other two branches of the European Union, arguing they lacked authority to conclude the data-sharing agreement.

David Henderson, a spokesman for the Association of European Airlines that includes about 30 major airlines, said the announcement "generates a great deal of uncertainty ... It is unclear what will happen."

Dutch European Parliament member Sophie in 't Veld said the opinion of Advocate General Philippe Leger was "so clear it will be difficult to ignore." Governments "take all kinds of tough measures which are not well thought out and have insufficient democratic legitimacy," she said.