Telegrams sent by the British security service led to the "extraordinary rendition" of two UK residents now in Guantanamo Bay, BBC News has learned.
Eight UK residents are thought to be held at Guantanamo Bay
Flight details sent to US authorities allowed Bisher al-Rawi and Jamil al-Banna to be arrested in Gambia.
The UK government has always said it opposes "extraordinary rendition" - secret flights taking terror suspects for interrogation in other countries.
The Foreign Office denies requesting the men's detention.
Mr al-Rawi and Mr al-Banna were arrested at Gatwick airport in November 2002, BBC2's Newsnight has learned.
British intelligence then sent US authorities a telegram saying one of them had been carrying an object that could have been used as part of an improvised explosive device.
The men were later released after MI5 found the device to be an innocent battery charger - but this time the US authorities were not informed.
The following week the men continued their journey - a business trip to Gambia, west Africa.
British intelligence then sent the US authorities a second telegram reminding them of the previous telegram, giving the men's flight details and saying they were associates of radical Muslim cleric Abu Qatada.
The telegram said: "This information is being communicated in confidence... should not be released without the agreement of the British government. It is for research and analysis purposes only and should not be used as the basis for overt or covert action."
But the men were arrested at the airport in the Gambian capital, Banjul, along with Mr al-Rawi's brother Wahab, a British citizen, who was there to meet them.
Their lawyer Brent Mickum told BBC News: "They were taken out in chains and hooded... to separate rooms, where there were seven or eight individuals all of whom were dressed completely in black and wearing black masks."
Wahab, who was later released, said that when he asked to see a representative of the British authorities in Gambia he was told: "Who do you think ordered your arrest?"
Mr Mickum added: "Their clothes were cut off... nappies were put on them. Then they were taken in chains to a jet."
The two men were flown to a CIA facility in Afghanistan known as the "Dark Prison", where the conditions were "hellish", Mr Mickum said.
"It was completely dark, they could not see anything, there was no light, they could not tell day from night.
"There were speakers blaring music 24 hours a day that made sleep almost impossible.
"They could hear screams on occasion from other prisoners."
In early 2003 the men were flown from Bagram airbase, north of Kabul, to Guantanamo Bay.
The telegrams proving Britain's involvement in the Gambia arrest emerged during a High Court battle in which the men have demanded the government act on their behalf to secure their release from Guantanamo Bay.
However, it is not clear if the UK government knew what would happen to them after they were arrested.
The Foreign Office said in a statement: "We can confirm that the UK did not request the detention of the claimants in The Gambia and did not play any role in their transfer to Afghanistan and Guantanamo Bay."
Businessman Mr al-Rawi, who is in his late 30s, has always maintained that he had contact with Abu Qatada which was "expressly approved and encouraged by British intelligence".
He said intelligence staff had told him that what he was doing was fine, and they would help him if he ever ran into trouble.
Last week, the UK government said it would act for Mr al-Rawi alone, after it emerged in court that he had co-operated with MI5, helping pass messages to Abu Qatada.
Mr al-Rawi, an Iraqi citizen with UK residency, was reportedly sent to England in 1985 after his father was arrested by Saddam Hussein's secret police.
Mr al-Banna is a Jordanian refugee who had been living in north-west London.
Both men deny any involvement with Islamic terrorism.
They are among at least eight UK residents still thought to be held at the US-run camp in Cuba.
On Tuesday, the all-party parliamentary group on "extraordinary renditions" will see and discuss copies of the Gambia telegrams, and hear from witnesses.
The group's chair, Conservative MP Andrew Tyrie, said: "What worries me very deeply is that we might have handed over people to the Americans knowing that these people might then have been maltreated."