AIR travel from Europe to the United States could be thrown into chaos after a key transatlantic terrorism measure was declared illegal by a senior European Union lawyer.

Philippe Léger, the Advocate-General of the European Court of Justice, the EU’s supreme court, called for the annulment of an agreement requiring EU airlines to give US authorities access to a wide range of confidential data on passengers before they travel.

Although all 25 EU governments and the European Commission gave full backing to the agreement, aimed at helping US authorities to investigate terrorists while protecting passenger confidentiality, M Léger said that it lacked an “adequate legal basis”.

The Advocate-General’s advice is not binding on the court, but is followed in about 80 per cent of cases.

The court is due to make its formal ruling next year, and will have to be obeyed by the commission and all of the governments.

A European Commission official said: “Everyone will lose. The whole system — all the safeguards put in place by the commission — will collapse.

“If it is cancelled, it brings us back to square one, which means that each airline is alone in the face of demands from the US.”

He also said that the annulment of a policy that governments and the commission had agreed “would seriously weaken the EU in international negotiations”.

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

Under the agreement, reached last year, European airlines give US authorities access to 34 items of confidential data about each passenger, including name, address, contact numbers, credit card details and connecting flights.

However, it does not include information on more sensitive issues such as the religion of a passenger, by, for example, giving details on who has ordered a halal meal.

Plane tickets from Europe to the US include a warning that confidential passenger data will be passed on to US authorities.

The US government demanded this data as part of its counter-terrorism strategy after the attacks on September 11, 2001. Airlines that refuse face fines or even a ban on entry to the United States. However, airlines were also warned by European data protection registrars that if they did pass on the information to the US they would be breaking EU laws on data confidentiality.

To break the impasse, the EU and national governments reached a deal with the US which, they insisted, met concerns about confidentiality.

However, if the court follows the Advocate-General’s ruling and cancels the agreement, airlines and passengers will be caught in the same legal trap between the EU and US.

Timothy Kirkhope, the leader of the Conservatives in the European Parliament, said: “Binning this accord would be an unnecessary and retrograde step which could potentially undermine the international fight against terrorism.

“If this agreement ends up being scrapped the US response could cause serious difficulties for EU airlines flying to America.”

The challenge in the European Court of Justice was mounted by the European Parliament, which felt that it had not been sufficiently consulted by the national governments and the commission.

Graham Watson, the leader of the Liberal Group in the European Parliament, said: “This is a victory for liberals and democrats. Part of our original criticism to the agreement was that it did not contain sufficient safeguards for data protection of EU citizens.”

A spokesman for the British Government, which currently holds the rotating presidency of the EU, said: “We don’t agree with the opinion but will await the [high court] decision.”

The US also said that it would wait for the formal decision before responding.

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