A report from Swiss MP Dick Marty for human rights watchdog the Council of Europe says a group of countries acted as "staging posts" in the transfer by American authorities of men wanted for questioning.
They include Britain, Germany, Spain, and Turkey, who co-operated in the running of so-called "rendition" flights - the covert transport of prisoners for questioning in countries where, it is claimed, many faced torture.
A preliminary report by Mr Marty earlier this year said European governments were almost certainly aware of the CIA's secret prisoner flights via European airspace or airports.
Now, at the end of a seven-month inquiry, the final report says it is now clear that "authorities in several European countries actively participated with the CIA in these unlawful activities".
Mr Marty warns that the inquiry has still not established the whole truth.
But he condemns what he calls a "spider's web" of US rendition flights as "utterly alien" to the concept of basic human rights.
Allegations that special American flights transported terrorist suspects to Europe to be questioned were first raised in the Washington Post last November.
The paper said the CIA had been running interrogation centres in Eastern Europe, Afghanistan and Thailand, and that more than 100 people had been sent to the so-called "black sites" since they were set up following the September 11 2001 terrorist attacks on America.
Mr Marty said his inquiry had identified the "rendition" of more than one hundred prisoners "affecting Europe".
Mr Marty described the system as "the outsourcing of torture", by relocating individuals for questioning in countries which do not necessarily follow human rights codes.
But his report has not delivered any firm evidence of the existence in any European country of secret CIA detention centres, said to have been set up since the 9/11 attacks.
However, it states: "It is now clear, although we are still far from having established the truth, that authorities in several European countries actively participated with the CIA in these unlawful activities."
And it emphases that suspicions remain of secret CIA detention centres in Romania and Poland - allegations both countries have denied.
Some European countries, while not actively involved in rendition flights, "ignored them knowingly or did not want to know".
There were "corroborated facts" strengthening the presumption that landing points in Romania and Poland were detainee drop-off points near to secret detention centres, said the report.
Today Mr Marty, unveiling the details in Paris, commented: "Even if proof, in the classical meaning of the term, is not as yet available, a number of coherent and converging elements indicate that such secret detention centres did indeed exist in Europe."
Those elements warranted further investigation, he added.
Mr Marty said he used evidence from national and international air traffic control authorities, as well as sources inside intelligence services, including in the United States, to compile a detailed picture of a global system of secret detentions and unlawful transfers - including new analysis revealing what he called "rendition circuits".
He named seven countries which he said "could be held responsible, in varying degrees, which are not always settled definitively, for violations of the rights of specific individuals".
They were the UK, Germany, Italy, Sweden, Bosnia and Herzegovina, Turkey and the former Yugoslav Republic of Macedonia.
Seven more colluded, "actively or passively", in the detention or transfer of unknown persons.
Washington has never denied moving terrorist suspects to other countries for questioning, but does deny allegations of torture, and of deliberately picking centres in eastern Europe and beyond, outside the US human rights jurisdiction.
Mr Marty's report is now due to be debated by the Strasbourg-based Council of Europe's Parliamentary Assembly later this month.
The Assembly brings together 630 MPs from the 46 Council of Europe member states, which include all 25 EU member states.
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