Minutes after the vote, a spokesperson from the Irish ministry of justice said that justice minister Michael McDowell had the intention of testing its judicial bearing, claiming it should not fall under the EU's so-called first pillar.

First pillar decisions are made by majority voting between member state governments, with a necessary consent by a majority of the European Parliament, but security-related justice decisions are usually made under the so-called third pillar, where unanimous consent by member states is necessary and parliament can only issue an opinion.

Mr McDowell has insisted that Ireland retain its veto in justice matters, and will consult the Irish attorney general about how to proceed with an appeal to the European Court of Justice (ECJ).

The ministry of justice in Slovakia, one of three countries who voted against the proposal in the council earlier this month, said that Slovakia agreed with the content of the directive, but also objected to placing it under the first pillar.

By contrast, an official of the Swedish ministry of justice, one of the member states keenest on introducing a data retention law, indicated that if the decision had not been placed in the first pillar it would instead have been taken before the ECJ by the commission and the council.

Arguments over pillars

The Irish and Slovak position on Wednesday received unexpected support from within the parliament - normally calling for more, not less, influence in EU decision-making - with MEPs from the green group saying the decision had been a "false" first pillar decision in any case.

"The council - member state ministers - have acted as if it was a first pillar decision, concluding a back-door deal with the two major groups in parliament that these would not post amendments to the council decision", Green MEP Carl Schlyter told EUobserver.

"Citizens have a right to know who is in charge of legislation, and these matters should not be tossed around between pillars", he said, adding he welcomed Ireland's initiative and a legal review of the legislation at the ECJ.

Liberal German MEP Alexander Alvaro argued along the same lines.

"By voting as we [the Parliament] did today we create a precedent where Council need only say 'jump!' and Parliament cries 'how high?'", he said.

Mr Alvaro added he will look into the possibility of taking the directive before the German High Court, suspecting that the court would "question very strongly if this is in line with its constitution."

MEPs cannot go to court

Several MEPs have argued that the data retention directive is in breach of the European Convention on Human Rights, which sets out to protect privacy and states that data should not be retained for "more than necessary."

The MEP Eva-Britt Svensson from the GUE / Nordic Left group indicated that a possible way to go would be to take the matter before the European Court of Human Rights in Strasbourg.

But a spokesperson for the France-based Council of Europe, the 46 member state political watchdog which guards the European Convention on Human Rights, reminded MEPs that only victims who suffer the consequences of a decision can take a case to the council, and not political groups or organisations who are against a political decision.