Two former Libyan detainees, Sami Al Saadi and Abdel Hakim Belhadj, at the centre of claims that British spies were involved in rendition and torture are launching legal proceedings against the former director of counter-terrorism at MI6.
The claims from the Libyans were deemed so serious by police and prosecutors that an immediate criminal inquiry was ordered by Scotland Yard earlier this month, scuppering Sir Peter Gibson's inquiry into the actions of British spies.
The two men are now launching a civil action against Sir Mark Allen, claiming that evidence of the UK's role in the couple's rendition is detailed in a number of documents held by the Libyan security services, which came to light after the fall of Colonel Muammar Gaddafi's regime.
Sir Mark, who was the director of counter-terrorism at MI6 until he left in 2004, has been notified of the intended legal proceedings against him, the pair's lawyers at Leigh Day & Co said.
The letter of claim seeks Sir Mark's response to allegations that he was complicit in torture and misfeasance in public office, as well as negligence, and claims damages from him personally for the trauma involved.
Also known as Abu Abd Allah Sadiq, Mr Belhadj, 45, a Libyan rebel commander who was living in exile in Beijing, China, says he was tortured after being detained with his wife in 2004 en route to the UK where they were trying to seek asylum.
The civil action stems from the discovery of a letter purportedly from Sir Mark to Moussa Koussa, head of Gaddafi's intelligence agency, dated March 18 2004.
In it, Sir Mark is said to pass on thanks for helping to arrange Tony Blair's visit to Gaddafi, writing: "Most importantly, I congratulate you on the safe arrival of Abu Abd Allah Sadiq.
"This was the least we could do for you and for Libya to demonstrate the remarkable relationship we have built over the years."
Sapna Malik, who is representing the men and their families, 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."
Cori Crider, legal director of the human rights group Reprieve, said the two Libyans 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 (Sir) Mark Allen's name all over them - yet we've not had a word out of him since they became public last year," she said.
"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."