Tony Blair wrote a letter of apology to Libyan leader Muammar Gaddafi in 2007 for the UK failing in its attempt to send two political exiles back to the North African country.
The letter, written on April 26, 2007, was recovered from Libyan government offices following the 2011 revolution that ousted the dictator.
It is now being used by a team of London lawyers who are bringing damages claims on behalf of a dozen Gaddafi opponents who claim they were targeted by the UK and Libya's counter-terrorism agencies.
In the letter, reported in The Guardian, Blair informs Gaddafi that the UK was about to fail in its attempts to deport two Libyans allegedly linked to the Libyan Islamic Fighting Group, which opposed the government.
He also thanked Gaddafi for the "excellent co-operation" between the two countries.
The Guardian said that papers recovered from Libya were now forming the basis of the damages claim being brought by six Libyan men, the widow of a seventh, and five British citizens of Libyan and Somali origin against the British government.
They are alleging false imprisonment, blackmail, misfeasance in public office and conspiracy to assault.
Following Gaddafi's decision to abandon weapons of mass destruction, the relationship between Libya and the UK thawed and it is known that the two countries had co-operated on intelligence matters.
A spokesman for Mr Blair said: "There is nothing secret about the fact that the UK and the then Libyan government co-operated in the fight against terrorism. It was public. Or that the then Libyan government gave up their chemical and nuclear weapons programme and that this happened in 2003 after negotiation.
"At that time the UK was fighting a serious terrorist threat aimed at its people and its soldiers. None of that means the UK Government accepted or condoned torture."