Two Libyans are to sue the former counter-terrorism director of British spy agency MI6, claiming he played a key role in their rendition to Moamer Kadhafi's Libya, their lawyers said Tuesday.

Abdelhakim Belhaj, who became Tripoli's military commander after the Libyan leader was ousted in last year's revolution, and fellow Kadhafi opponent Sami al-Saadi claim British complicity in their capture and subsequent torture.

They are taking action against Mark Allen, MI6's counter-terrorism chief at the time, after documents emerged suggesting his direct involvement.

"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," said Sapna Malik, a lawyer at Leigh Day and Co.

"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."

British police announced this month that they would investigate the Libyans' claims, and MI6 director Mark Sawers said the agency would fully cooperate.

Files unearthed from Kadhafi's archives last year said Belhaj was captured by the CIA in Bangkok in 2004 and with British help was forcibly returned to Libya, where he was jailed in the notorious Abu Salim prison.

In a letter to Kadhafi's intelligence chief Moussa Koussa, dated March 18, 2004, Allen purportedly writes: "I congratulate you on the safe arrival of Abu Abd Allah Sadiq (Mr Belhadj)."

He continues: "This was the least we could do for you and for Libya to demonstrate the remarkable relationship we have built over the years."

Saadi meanwhile claims British agents helped detain him in Hong Kong in 2004 and return him to Libya, where he was subjected to years of torture.

"Sir Mark has now been notified by both of the men that they intend to take legal proceedings against him," said a statement from Reprieve, a legal charity which is helping the Libyans.

"A letter of claim, sent by their UK lawyers Leigh Day and Co, seeks his response to allegations that he was complicit in torture and misfeasance in public office; and to examine his exact role in the rendition of both men, as well as claiming damages from him personally for the trauma involved."

Allen left MI6 later in 2004 to work for energy giant BP, Reprieve said.