EURO MPs are to investigate allegations that the United States is operating secret CIA prisons on European soil and illegally transferring terrorist suspects between countries.

The inquiry, which will begin next year and looks certain to sour relations between Washington and Brussels, was approved by the European Parliament’s political group leaders last night after mounting pressure from backbenchers.

The investigation could embarrass Poland and Romania, which have been accused of acting as bases for interrogating terrorist suspects or as a transit point for moving them to other countries. Any violation of human rights by an existing EU member such as Poland could see it stripped of its voting rights, while Romania, which aspires to join the Union in January 2007, could find its hopes dashed.

The parliamentary inquiry, which will have three months to present its initial findings, is expected to have an unusually wide remit. It is likely be asked to examine alleged abuses such as kidnapping, false arrest or illegal transfers not only within the European Union, but also on the territory of all countries with which it is associated.

The Euro MPs will co- operate with the separate investigation, headed by Dick Marty, a Swiss senator, that the 46-nation Council of Europe began last month. Mr Marty delivered an interim report stating that his work had “reinforced the credibility of the allegations concerning the transfer and temporary detention of individuals, without any judicial involvement, in European countries”. Frank-Walter Steinmeier, the German Foreign Minister, denied yesterday that Berlin had colluded in the kidnapping and interrogation of terror suspects by the CIA.

Herr Steinmeier said that he and other members of the former government under Gerhard Schröder had faced “irresponsible speculation and suspicions” relating to the kidnapping of Khaled al-Masri, a Lebanese-born German suing the CIA for holding him in an Afghan jail for five months.

The case, which also involves criticism that Berlin has been allowing the CIA to fly terrorism suspects through German airspace, has marred Angela Merkel’s attempts to repair German relations with the United States since becoming Chancellor this year.