Euro MPs are to launch an investigation into 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 tonight after mounting pressure from backbenchers.

The investigation could embarrass Poland and Romania, which have been accused of acting as a base 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 could see it stripped of its voting rights, while Romania, a candidate country which is hoping to join the Union in January 2007, could find its accession 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 just within the European Union, but also on the territory of all countries with which it is associated.

It will consider whether the activities contravene European law and the European Charter of Human Rights and assess the potential consequences of any illegal and unlawful practices. However, while its members may travel freely and will be able to request that witnesses give evidence, they will not have the power to insist they do so.

The Euro MPs will co-operate with the separate investigation headed by Dick Marty, a Swiss senator, that the 46-nation Council of Europe launched last month.

Mr Marty reported yesterday that his month-long study "reinforced the credibility of the allegations" surrounding the CIA - although he said that any prisoners formerly held in secret sites had by now probably been transferred to North Africa.

Mr Marty said that he had unearthed "clues" that both Romania and Poland were implicated, perhaps unwittingly, in the CIA's interrogation of terrorism suspects.

"Legal proceedings in progress in certain countries seemed to indicate that individuals had been abducted and transferred to other countries without respect for any legal standards. It had to be noted that the allegations had never been formally denied by the United States," he said.

"While it was still too early to assert that there had been any involvement or complicity of member states in illegal actions, the seriousness of the allegations and the consistency of the information gathered to date justified the continuation of an in-depth inquiry.

"If the allegations proved correct, the member states would stand accused of having seriously breached their human rights obligations to the Council of Europe."

A leading British legal expert yesterday urged the Government to find out more about the CIA's alleged use of British airspace to carry out renditions, saying Britain could be breaking the law if it fails to ask whether the practice leads to torture.

Simply relying on American assurances that aircraft that are being used to carry terrorists to interrogation centres and which stop over in Britain are not so called "torture flights" is not enough to comply with legal obligations, according James Crawford, professor of international law at Cambridge University.

In a legal opinion for MPs investigating the flights, Prof Crawford wrote that the question that must be asked is "whether torture is likely to take place if a person is transported, irrespective of whether or not the Government claims that the answer is no, or what its hopes or beliefs may be.

"Where governments are using public power to transfer persons at risk to a given country, in circumstances where earlier practices support credible allegations of torture in that country, mere assurances by the government, unaccompanied by other action, will be insufficent," he concluded.

Growing unease about the CIA's rendition programme, which is believed to have transported 3,000 terrorist suspects for interrogation around the world since 2001, has prompted investigations in Finland, Hungary, Iceland, Italy, Norway, Poland, Portugal, Romania, Spain and Sweden.