A European Parliament committee has approved a report which says EU states knew of secret CIA flights over Europe.
The UK, Germany and Ireland saw the highest volume of flights
The report says the governments also knew of the abduction of terror suspects by US agents and the US's use of clandestine detention centres.
But it says claims that the CIA had a secret prison in Poland are unproven.
The report, which goes to a vote of the full parliament next month, also says the UK, Italy and Poland were reluctant to co-operate with the investigation.
EU foreign policy chief Javier Solana and EU counter-terrorism co-ordinator Gijs de Vries are accused of failing to reveal all they knew to the special parliamentary committee.
Surrounded by double-lined fences, security cameras and thick pine forest, visitors are not welcome
The committee's conclusions, published in draft form November, are similar to those of a separate Council of Europe investigation published last year, which talked of a "global spider's web" of secret flights.
The report says more than 1,000 covert CIA flights crossed European airspace or stopped at European airports.
The volume of flights was greatest in the UK, Germany and Ireland, it adds.
The MEPs say the UK, Poland, Italy, Germany and seven other countries knew of the flights and the detention programme, which may have violated EU human rights law.
US President George Bush admitted in September that terror suspects had been held in CIA-run prisons overseas, but he did not say where the prisons were located.
A BBC investigation last year revealed that a well-known CIA Gulfstream plane, the N379P, had made several landings at Szymany airport in northern Poland in 2003.
The airport's flight log also showed that a Boeing 737 had flown direct from Kabul to the airport, which is not far from a Polish intelligence base in the village of Stare Kiejkuty.
The committee's original draft report stated that: "In the light of... serious circumstantial evidence, a temporary secret detention facility may have been located at the intelligence training centre at Stare Kiejkuty."
That sentence has now been amended, to read: "It is not possible to acknowledge that secret special centres were based in Poland."