Reports said Britain, the current president of the EU, had been asked to draft the letter at a 21 November EU foreign ministers' meeting in Brussels.
The "Washington Post" newspaper reported earlier this month that the CIA had been interrogating suspected Al-Qaeda prisoners at secret facilities.
Human Rights Watch said it had evidence suggesting that the CIA had transported suspected terrorists captured in Afghanistan to Poland and Romania. Both countries have denied it.
The Parliamentary Assembly of the Council of Europe says it has launched an investigation into the allegations.