31 January 2012 Last updated at 06:08 ET
Two men who say they were rendered to Libya with the help of the UK have begun an action to sue one of Britain's most senior former MI6 officers.
Libyan dissidents Sami al-Saadi and Abdel Hakim Belhadj allege that Sir Mark Allen was complicit in their rendition and torture.
Lawyers for the pair served papers on Friday, the first step in a civil action for damages.
Sir Mark has declined to comment on the allegations and legal action.
Mr al-Saadi and Mr Belhadj say they were handed over to Col Muammar Gaddafi's regime in 2004.
The Metropolitan Police has already begun its own investigations into the allegations.
The pair had already begun a legal action against the UK government, alleging that it aided and abetted their ill treatment.'Dark episode'
Both men say that they were illegally detained and flown to Libya with the help of the UK.
Their wives and children were also rendered to Libya. Mr Belhadj says that his wife was also subjected to ill-treatment.
But in a highly unusual move, lawyers for the men have launched a civil damages claim against Sir Mark Allen, MI6's former head of counter-terrorism.
Lawyers for the men say they want to sue Sir Mark because his name appears in documents discovered in Tripoli during Gaddafi's downfall.
They say the documents include a letter allegedly written by Sir Mark in March 2004 to the colonel's then spy chief, Moussa Koussa.
"I congratulate you on the safe arrival of Abu Abd Allah Sadiq [Mr Belhadj]," says the letter.
"This was the least we could do for you and for Libya to demonstrate the remarkable relationship we have built over the years. I am so glad.
"The intelligence on Abu Abd Allah was British. I know I did not pay for the air cargo. But I feel I have the right to deal with you direct on this."
Sapna Malik, the men's solicitor at firm Leigh Day & Co, said Sir Mark would be sued for complicity in torture, misfeasance in public office and negligence.
She said: "We are taking this unusual step of preparing a legal action against an individual as the documents we have in our possession suggest Sir Mark was directly involved in the unlawful rendition of our clients and their families.
"The documents which have so far come to light raise serious questions to answer, particularly in light of the horrendous treatment to which our clients were subjected. There must be full accountability for this dark episode."
Sir Mark, who left MI6 later in 2004 and joined BP, has declined to comment on the allegations and the legal action.
Earlier in January, the CPS and Metropolitan Police made a joint statement confirming that the allegations raised by the two Libyan men were so serious that they must be investigated before a planned judge-led inquiry into allegations of rendition and torture.
That decision prompted the government to scrap the inquiry, although it has pledged to launch another one after the end of criminal investigations.
Cori Crider from legal charity Reprieve, which is working on the case, said: "Abedelhakim Belhadj and Sami al-Saadi - and their families - deserve an explanation for the appalling abuse they suffered. The documents showing British involvement in their rendition to Libya appear to have Mark Allen's name all over them - yet we've not had a word out of him since they became public last year.
"We need to know whether MI6's plan to deliver Gaddafi's opponents into his hands was authorised from the top. If it was, let Sir Mark bring that defence, and let's hear who signed off on this shameful affair."