Eleven police forces were today threatened with legal action if they fail to investigate allegations that UK airports are being used as secret stop-overs by CIA jets transferring terror suspects to torture camps.

The human rights group Liberty has called on the chief constables of forces from Prestwick, near Glasgow, to Bournemouth to investigate claims that the airports are facilitating kidnap and torture - which is illegal under British and European Union law.

The move follows international concern over the CIA's failure to confirm or deny suggestions that it has illegally abducted terrorist suspects and flown them between a network of clandestine detention centres - so-called "black sites" - for interrogation under torture.

The potentially devastating allegations threaten to overshadow a tour of European capitals by Condoleezza Rice, the United States secretary of state, next week. Britain, in its role as president of the EU, has demanded "clarification" from the White House.

Shami Chakrabarti, the director of Liberty, today told the BBC: "The allegations, the suspicions and the circumstantial evidence gives serious enough concern that we are asking chief constables to do their duty and investigate.

"If they will not investigate then I'm afraid we will have to seek, unhappily, the address of the courts." The forces have been given 14 days to respond to the letter, she said.

The process of "outsourcing" interrogation, known as extraordinary rendition, was developed by the CIA in the mid-1990s with the approval of the Clinton administration in an attempt to dismantle groups affiliated to al-Qaeda.

Finland, Hungary, Iceland, Italy, Norway, Poland, Portugal, Romania, Spain and Sweden have already opened investigations into the clandestine stop-offs.

Flight records show that at least 210 private jets carrying detainees apparently leased by shell companies attached to the CIA have stopped over in the UK since September 2001.

Liberty said that the practice was in breach of British and international law, and European and United Nations human rights conventions, which forbid complicity in torture.

It has written to the chief constables of Bedfordshire, Cambridgeshire, Dorset, Essex, Hampshire, Kent, the Metropolitan Police, the Ministry of Defence police, Sussex, Thames Valley and the West Midlands police forces.

The airports where the flights are alleged to have landed include Biggin Hill in Kent, Birmingham, Bournemouth, RAF Brize Norton in Oxfordshire, Farnborough, Gatwick, Heathrow, Luton, Mildenhall in Suffolk, RAF Northolt in north London, Stansted and Prestwick.

The Washington Post reported that the CIA was holding suspects from Osama bin Laden’s al-Qaeda group in prisons in eight countries including Thailand, Afghanistan and "several democracies in eastern Europe". The "black sites" are characterised by a less stringent attitude towards human rights and torture than prisoners would expect on American soil.

Humans Rights Watch, a United States-based independent watchdog, has said that it is "practically convinced" that such centres existed in Poland and Romania.

Chris Mullin, the Labour MP who is a member of the all-party parliamentary committee on rendition, told the BBC that while he was not aware that the British Government was knowingly complicit in the practice, it displayed a "lack of curiosity".

He said: "There's no doubt some sort of secret gulag exists which is controlled by the Americans into which people disappear for months at a time. And there's also no doubt that the Americans have for some time been franchising out torture to countries that are rather less scrupulous than ourselves, and indeed the Americans, about the use of torture."

Ms Chakrabarti said: “What distinguishes democrats from dictators and terrorists, including the people who are holding a British hostage in Iraq today? What distinguishes us is ultimately our abomination of torture. That’s why we, as democratic people who believe in human rights, will not tolerate it.”

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