Reports claim al-Qaeda members are being held in clandestine jails
The European Union's top justice official has warned that any EU state found to have hosted a secret CIA jail could have its voting rights suspended.
Franco Frattini said the consequences would be "extremely serious" if reports of such prisons turned out to be true.
This comes amid an EU investigation into claims that US secret agents ran clandestine jails in eastern Europe.
In the case of Romania, a senior Euro MP has questioned whether its accession to the EU should go ahead as planned.
The US has refused to confirm or deny the reports of secret jails, which surfaced in the US earlier this month.
Speaking at a news conference in Berlin, the EU Justice Commissioner said he would call for tough penalties against any involved state.
"I would be obliged to propose to the Council [of EU Ministers] serious consequences, including the suspension of voting rights in the council," he said.
He said a suspension of voting rights would be justified if any country is found to have breached the bloc's founding principles of human rights, democracy and the rule of law.
Such a move would be unprecedented.
A diplomatic source said that to suspend a member state's voting rights in the Council of the European Union, the other 25 member states would have to vote unanimously to take such a step, which would be unlikely to happen in practice.
Mr Frattini said the Bush administration had asked for more time to deliver a response to the accusations after a senior commission official formally raised the issue on a visit to Washington last week.
"Right now, there is no [US] response," he said.
The allegations that the CIA held al-Qaeda suspects in secret prisons in Eastern Europe were first reported in the Washington Post on 2 November.
According to civil liberties group Human Rights Watch, the jails are based in Romania and Poland.
Mr Frattini said Romania's Interior Minister, Vasil Blaga, told him there were no such prisons in his country.
Elmar Brok, the chairman of the European Parliament's Foreign Affairs Committee, said if Romania had secret prisons the EU ought to reopen accession talks, despite the treaties which had already been signed.
Mr Frattini said it was "very, very important to get the truth", but he cautioned that it was "impossible to move only on the basis of allegations".
Meanwhile, Germany's Foreign Minister Frank-Walter Steinmeier is expected to raise the issue of "secret" prisons in talks with US Secretary of State Condoleezza Rice in Washington on Tuesday.
Speaking on a visit to the United Nations in New York, Mr Steinmeier said he presumed "the seriousness of these [accusations] is being recognised" by the Bush administration.
The UK Foreign Office has confirmed that Britain will be writing to the US, on behalf the EU, to clarify the reports of secret prisons, which were reportedly set up after the 11 September 2001 attacks.
Spain, Sweden and Iceland are looking into separate reports that CIA planes stopped in their territory while transporting terror suspects.
The European Council has appointed Swiss Senator Dick Marty to investigate what he called the suspicious movement patterns of flights in the region.