As Tony Blair faces calls to make a full statement on how much he knew about the US using UK airports to transport terror suspects, BBC News examines the accusations about the flights.
A memo published in the New Statesman has reignited the row over the so-called US "terror flights".
Critics claim the US routinely flies suspects to secret detention centres where they could be tortured.
The leaked Foreign Office memo, written on 7 December last year, said officials had discovered two cases from 1998 - one accepted and one denied - of US flight requests. But it suggested there could be more.
The document advises ministers to avoid detailed questions on the so-called "rendition", or transfer of a terror suspect from one juridisction to another outside normal legal processes - by the US.
A Foreign Office spokesman said Jack Straw had already made clear the UK had not agreed, and would not agree, to help transfer people to places where there were "substantial grounds to believe they would face a real risk of torture."
A Guardian article published on 12 September 2005 claimed 19 British airports and RAF bases, including Biggin Hill, Belfast Birmingham, Bournemouth, Brize Norton, Farnborough, Gatwick, Heathrow, Luton, Mildenhall, Northolt, and Stansted, have allowed a 26-strong fleet of CIA or CIA-chartered jets to land temporarily.
The article also said these aircraft had flown into the UK at least 210 times since September 2001, an average of one flight a week.
The favourite destination was Prestwick, where CIA aircraft have flown into and out from more than 75 times, the article said, Glasgow had seen 74 flights, and RAF Northolt 33.
Amnesty International has published the details of CIA flights it says used UK airfields to refuel just hours after transferring detainees to countries where they risked torture.
The allegations concern a Gulfstream V turbojet, then registered as N379P.
The organisation says between 2001 and 2005 the plane made at least 78 stopovers at UK airports while en route to or from destinations including Baku, Dubai, Cyprus, Karachi, Qatar, Riyadh, Tashkent, and Warsaw.
The allegations are as follows:
- On 23 October 2001, witnesses saw Jamil Qasim Saeed Mohammed being bundled on board a Gulfstream V, registration N379P, by a group of masked men at Karachi airport in Pakistan.
The plane flew the terror suspect to Jordan.
The following day, it flew to Glasgow Prestwick to refuel, then back to Dulles International near Washington DC.
Amnesty International says it has repeatedly requested information from the US authorities about the current whereabouts and legal status of the terror suspect, but has received no reply.
- On 18/19 December 2001, the plane took Ahmed Agiza and Mohammed al-Zari from Sweden to Cairo.
It had made several trips between Cairo and Prestwick earlier in the month, and stopped to refuel at Prestwick after leaving the two detainees in Cairo, where they were reportedly tortured.
- On 12 January 2002, the plane took Muhammad Saad Iqbal Madni from Jakarta, Indonesia, to Cairo, Egypt.
When the plane left Cairo, it flew to Prestwick to refuel.
The terror suspect has since been returned to US custody, and is currently being held at Guantanamo Bay.
He told other detainees, who have since been released, that he was tortured in Egypt by having electrodes placed on his knees.
Other detainees have said in the last month that he is in poor condition and "at risk of losing his mind".
It also appears his bladder was damaged during interrogation.