Controversial proposals to create a team of elite "Euro-prosecutors", which critics say will undermine Britain's judicial independence, take a step closer to fruition this week.
Brussels officials meet on Tuesday to discuss the plan for prosecutors to tackle fraud, money-laundering, drug-trafficking and other cross-border crimes.
But Conservatives say the scheme will be the forerunner of a single legal system for Europe, known as corpus juris. A senior lawyer said yesterday that the plan contained "completely unacceptable" proposals.
Some in the legal profession are particularly concerned that a European public prosecutor would have the right to detain suspected criminals for long periods without charge, in the face of ancient British rights enshrined in the law since Magna Carta.
A senior Conservative source said: "Our fear is that the Government will put its foot down on plans to harmonise tax rates, but, to buy fellow governments off, will agree to greater qualified majority voting or harmonisation in other areas.
"This could include issues such as legal reform. There is a worry that this is something the British government will have to compromise on if it wants to put its foot down in other areas."
The EU meeting, which will send its findings to ministers for discussion this month, comes amid renewed fears that some members of the European Union are continuing to press for greater integration.
Conservatives were furious when German foreign minister Joschka Fischer claimed a European federation led by Germany and France was the answer to the EU's problems.
Mr Fischer said in Berlin on Friday that the "finality" of European integration demanded "nothing less than a European parliament and a European government which really do exercise legislative and executive power within the Federation".
Downing Street said they were Mr Fischer's personal views and represented the feelings of only a minority. But the shadow foreign secretary, Francis Maude, said: "He has spectacularly blown the lid off Europe's superstate agenda. Unlike Tony Blair, Herr Fischer is honest about the direction in which Europe is heading."