Dick Marty, the Swiss politician looking into the case for the Council of Europe, the 46-nation human rights organisation, said in an interim report that US renditions, or extra-legal abductions, had challenged “the very foundation of the law-based state and its democratic foundation”.

“Individuals have been abducted, deprived of their liberty and all rights, and transported to different destinations in Europe, to be handed over to countries in which they have suffered degrading treatment and torture,” he said. “If governments resort to gangster-style methods, I say no,” he added.

The US maintains rendition is compatible with international law and that it does not transport people to countries where they would be at risk of torture. But Washington uses a more narrow definition of torture than do many European states.

Mr Marty said it was “highly unlikely that European governments or at least their intelligence services were unaware” of “hundreds” of CIA flights and more than 100 renditions on European soil or passing through European airspace.

“There is a great deal of coherent evidence pointing to the existence of a system of ‘relocation’ or ‘outsourcing’ of torture” by the US, said Mr Marty.

But he admitted that many of his findings – such as the number of likely renditions – were based on press reports and acknowledged that an in-depth study by a Romanian human rights organisation had not substantiated allegations that Romania had hosted a CIA prison.

Nor did he uncover any evidence of such sites in Poland, the other country that has come under most suspicion. He had received satellite pictures and flight records from official European agencies only this week and would now analyse them.

“The Marty report has more holes than a Swiss cheese,” said Denis MacShane, Britain’s former Europe minister, who attended the Council of Europe meeting in Strasbourg. “It is sad that Mr Marty refuses to accept the categorical denials of the Polish and Romanian governments.”

It is the behaviour of European states – rather than the actions of the US – that comes under the competence of the Council of Europe. Mr Marty criticised European governments for their reluctance to provide information and was backed by a call from the European Commission. Last week, the European parliament set up a parallel investigation, led by a 46-member committee.

In the UK, Mr Marty’s report fuelled renewed calls for the British government to broaden its own investigations into allegations that British airports may have been used as part of the “rendition” strategy.

The UK civil rights group Liberty is considering taking legal action against ministers on the basis that the Foreign Office’s investigations have been inadequate to allay public concern that the UK government might have been complicit in acts of torture.

“It is time for our [the UK] government to get its story straight – not about what it did not know, but what it is going to do about such serious alleged violations of human rights and UK sovereignty.”

Opposition parties say they remain unconvinced by UK foreign secretary Jack Straw’s statement to MPs that he had found no evidence of detainees being transported through the UK or British Overseas Territories since 11 September 2001.