An investigator looking into claims of secret CIA prisons in Europe today said that people were apparently abducted and transferred between countries illegally.

Swiss senator Dick Marty told a news conference that he believed the United States was no longer holding prisoners clandestinely in Europe but that claims about "extraordinary renditions" of prisoners, some of whom allegedly faced torture, had "credibility".

He said he believes any prisoners who were held in eastern Europe - there have been claims about secret prisons in Romania and Poland - were moved to North Africa in early November, when reports about the secret detention centres appeared in the Washington Post.

In a written report, Sen Marty said that information gathered so far "reinforced the credibility of the allegations concerning the transfer and temporary detention of individuals, without any judicial involvement, in European countries".

"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," he added in his findings presented in Paris to a committee of the Council of Europe, the continent's human rights watchdog.

Sen Marty is investigating claims that the CIA transferred prisoners through European airports to secret detention centres, actions that would breach the continent's human rights principles.

Poland and Romania have been identified by the New York-based Human Rights Watch as sites of possible CIA secret prisons, but both countries have repeatedly denied any involvement.

Sen Marty, in his report, added that it was "still too early to assert that there had been any involvement or complicity of member states in illegal actions".

He was critical of the United States for not formally confirming or denying the allegations. He said he "deplores the fact that no information or explanations" were provided by the US secretary of state, Condoleezza Rice, who faced repeated questions about the CIA prison allegations on her recent visit to Europe.

Sen Marty has requested air traffic log-books to try to determine flight patterns of several dozen suspected CIA aeroplanes. He has also requested satellite pictures of the Sczytno-Szymany airport in north-east Poland and the Mihail Kogalniceanu air base in eastern Romania, after they were identified by Human Rights Watch as possible sites of clandestine CIA detention centres.

After hearing Sen Marty's presentation, Tony Lloyd, a member of the Council of Europe committee, said, "The really difficult thing is the idea is that there is a kind of legal black hole in the middle of Europe."

Meanwhile today, a group of British MPs described as "worthless" the statements made by the foreign secretary, Jack Straw, that there were no records of the CIA flying terror suspects through the UK to face torture elsewhere.

There have been claims that CIA flights carrying detainees had gone through UK airports, including RAF bases, on more than 200 occasions since the September 11 2001 attacks on the US.

Mr Straw insists, however, that extensive research indicates that there have been no requests from the Americans for such flights to pass through the UK during the presidency of George Bush, which started in January 2001.

Sir Menzies Campbell, foreign affairs spokesman for the Liberal Democrats, is among a group of MPs who have set up the Parliamentary Group on Extraordinary Rendition, which today criticised Mr Straw's position.

The group said that an expert on international law who they had commissioned had said that Britain could not rely on US assurances that terror suspects sent to third countries do not face torture.

Professor James Crawford, of Cambridge University, said the UK must satisfy itself that "extraordinary rendition" of suspects does not put them at risk.

This afternoon, Mr Straw repeated his assurances that the UK was not complicit in torture and there was no evidence about any CIA flights.

He said there was no "secret conspiracy" and that unless people believed he and his US counterpart were lying, there was no truth in the allegations.

Mr Straw said there was no reason for a judicial-level investigation into the reports about renditions and said he did not know of the existence of any secret prisons in Europe.

Earlier, Andrew Tyrie, Conservative chairman of the Parliamentary Group on Extraordinary Rendition, said Mr Straw's words "should reassure nobody".

He said: "It is crystal clear that the UK must investigate allegations that it has been complicit in torture. Checking instances of the US requesting permission is simply derisory."