Police have launched an investigation into persistent claims that the CIA used British airports to fly terrorist suspects for torture in secret camps abroad.
Days after Tony Blair insisted he knew nothing about such "extraordinary rendition" operations, the move by the Association of Chief Police Officers (Acpo) threatens to embarrass the Government.
It is also the first attempt by the authorities to examine detailed claims that CIA flights have touched down more than 200 times in Britain.
Michael Todd, chief constable of Greater Manchester, is expected to review evidence collected by human rights campaigners and interview senior police officers from 10 forces across the country.
The investigation will attempt to establish whether there is evidence to back claims that CIA flights used British airports as stopping-off points while carrying terror suspects to secret detention camps around the world. It could develop into a criminal inquiry.
Mr Todd is due to meet campaigners in the new year to discuss his preliminary findings.
The move by the police came during a meeting yesterday with senior figures from the human rights group Liberty.
Shami Chakrabarti, the director of Liberty, said: "We are very pleased the police are taking these concerns seriously. If suspects are being taken through the UK on their way to face torture, there have been serious breaches of international and domestic law. We intend to help the police and call on individuals with any information to come forward."
Andrew Tyrie, the Conservative chairman of the parliamentary all-party group on rendition, yesterday welcomed the investigation and urged civil servants with knowledge of illegal rendition flights to report their evidence to their departmental heads.
Mr Tyrie said: "I'm pleased by this decision. There is increasing circumstantial evidence to suggest that extraordinary rendition may be taking place and that activity may well be illegal. I very much regret that the Government have done their best to walk on the other side of the street and have made no proper effort to investigate it at all."
He said: "It's the duty under the Civil Service Code for anyone to report to their permanent secretary information about possible unlawful activities, including breaches of any international treaties."
Sir Menzies Campbell, the Liberal Democrat foreign affairs spokesman, said: "This is a most welcome development and underlines the independence of the investigating authorities in the United Kingdom. If the law is being breached and there is evidence to establish this, prosecution should follow."
"If any use of British airfields is in breach of our obligations, there would not only be legal consequences but serious political ones as well."
An Acpo spokeswoman refused to disclose details of the investigation. She said: "It was a useful discussion and we will have a further meeting with Liberty in January."
A group of peace activists in West Sussex has submitted a formal request to Kenneth Jones, the county's chief constable, to supply details of US flights in and out of Gatwick airport.
The Committee Against Criminal Renditions has told him: "It is clearly apparent, from credible reports and information, that flights by certain American civil registered aircraft have transited the airport, which are involved in these activities."
It is calling for an investigation into whether "your force, or any other force or security service acting with your knowledge, has undertaken any investigation into the nature of these flights, their crew, passengers and purpose and what was the outcome of any such investigation".
A spokeswoman for Sussex police said: "We will be looking into the contents [of the claims] and will respond accordingly."Reuse content