For eurosceptics, the European Court of Justice ruling in September 2005 was like giving a child a loaded gun. It opened the way for the European Union to designate a new class of pan-European crimes, and how they should be punished.

In Britain there was an outcry. In future decisions taken in Brussels could be applied to the British courts, denying parliament the right to determine what constituted a crime and levels of sentencing.

Concerns grew when the European Commission interpreted the ruling as being far wider than the case at issue: environmental crime. It produced a list of offences it believed should also be covered by the new rules, including counterfeiting, money laundering and computer hacking.

The Commission's decision this week to create common criminal rules for environmental crimes is seen by some as a sign that Brussels will take full advantage of the court ruling to stealthily advance EU powers.

Franco Frattini, the EU's justice commissioner, is said by aides to be fully aware that the court has handed him a powerful legal weapon, but it is one that he will use with restraint.

"Member states are concerned about sovereignty on this issue and that was made very clear by justice ministers in 2005 after the court ruling," said one of Mr Frattini's aides.

He points to the fact that in the 18 months since the ruling, Mr Frattini has only acted to create common criminal standards in two areas: counterfeiting and now environmental crime.

"We don't see this as the beginning of a European criminal law or as a mandate to start writing a European criminal code," said a senior EU official.

But what is to stop zealous Brussels officials fulfilling the Napoleonic tendencies often ascribed to them by eurosceptics and laying down the law to member states?

The first limit was applied by the European court, which said the EU could only use criminal law to achieve its objectives when it could show that "effective, proportionate and dissuasive" penalties were essential to combating serious environmental offences.

In many cases where the EU makes law, member states may already apply criminal sanctions across the board. However, in cases - like environmental crime - where some countries only apply civil penalties, the Commission may be tempted to act to bring consistency.

Secondly, any attempt to designate such a euro-crime would still have to be approved by a qualified majority of EU member states and the European parliament. Mr Frattini has promised he would only make proposals in this area if he was already sure of widespread support.

Last night British lawyers said the 2005 European Court ruling did not appear to have had any significant practical consequences.

But the sceptics argue that these are early days. They fear that in time Brussels might forget the court's reminder that as a general rule "neither criminal law nor the rules of criminal procedure fall within the Community's competence".