CIA jets suspected of flying terrorist suspects to secret prisons for torture have landed at commercial British airports and received help from UK air traffic control, the authorities have admitted for the first time.
National Air Traffic Services (Nats) confirmed that three planes with CIA tail numbers have travelled through Britain "on a number of occasions".
MPs last night seized on the letter as the first formal acknowledgement that British authorities were aware that CIA flights associated with "extraordinary rendition" have travelled through UK airspace.
They said it showed that ministers could no longer claim they had no knowledge of CIA flights that have been linked to the policy of sending terrorist suspects for interrogation in countries that carry out torture.
Nats admitted it had provided a service to the flights after a number of Parliamentary questions to Transport ministers from Sir Menzies Campbell, acting leader of the Liberal Democrats.
The letter, written to Sir Menzies following orders from Transport minister Karen Buck, says that of four aircraft identified from records as having been used by the CIA, "three have received an ATC [air traffic control service] from Nats on a number of occasions in the past five years. We are not prepared to offer a number because we are not confident that such a number would be robust."
The planes are part of a ghost fleet of CIA jets that have been spotted at UK airports since 2001. Nats implies that they have travelled here frequently and may even have travelled under different call signs.
It said the flights may also have used airspace controlled by the Ministry of Defence. Defence ministers have been criticised for refusing to answer questions put down by Sir Menzies about how often the CIA jets have landed at military air bases. Defence minister Adam Ingram said "the information is not recorded centrally".
But the admission by the civil aviation service that CIA aircraft have used UK airspace is the first admission that the authorities and ministers are aware of the flights.
Tony Blair has claimed that he has no knowledge of so-called torture flights coming in and out of the country, and has refused to hold an independent public inquiry.
The flights have been associated with the practice of extraordinary rendition. which involves taking terrorist suspects to foreign prisons and secret jails in Europe. Seventy-six planes used by the CIA are believed to have made stops in Britain since 11 September 2001, at Prestwick, RAF Northolt, Luton and Glasgow.
Last night, Nick Clegg, a Liberal Democrat foreign affairs spokesman, said the letter was a significant admission. "It is significant that a public agency has confirmed the frequency of these flights through UK airports," he said. "More questions remain about their destination and what they contained."