Two Libyans who allege they were rendered to Tripoli by the US with the complicity of Britain have been interviewed by the Metropolitan Police over their claims.
Sami al Saadi and Abdelhakim Belhadj say they were forcibly flown back to Libya in 2004 in a joint operation by MI6, the CIA and Colonel Gaddafi's intelligence services.
Mr Saadi was accompanied by his wife and his four children while Mr Belhadj was accompanied by his pregnant wife.
Both men were imprisoned and tortured for years on their return.
Documents unearthed following the overthrow of the Gadaffi regime suggested that Sir Mark Allen, MI6 former head of counter-terrorism, and other British security service agents had conspired in the rendition.
Senior government figures including former foreign secretary Jack Straw are also potentially implicated in the rendition operation.
The Met and the Crown Prosecution Service announced a joint investigation in January saying that the allegations appeared "so serious that it is in the public interest for them to be investigated now."
It is understood the interviews with Mr Saadi and Mr Belhadj took place in Tunisia as the Met was denied access to Libya to investigate the rendition allegations.
Reprieve legal director Cori Crider said: "This is a major step in an historic investigation, but there's heaps still to do - most of it in Libya itself.
"The meetings with Mr al Saadi and Mr Belhadj had to be fixed elsewhere because the Libyans were taking so long to let the Met in on this case and of course it's where the families and other major witnesses still are."