A former SAS soldier will claim that British special forces are being used to detain suspects for extraordinary rendition.
Ben Griffin says the Government is "deeply involved" in the process, in which US forces have transported terror suspects around the world for interrogation.
He left the Army on moral grounds at the beginning of last year after three months in Baghdad, saying he disagreed with the "illegal" tactics of US troops.
Last week Foreign Secretary David Miliband told MPs in the Commons that two US rendition flights transporting terrorist suspects had landed on UK soil, but Mr Griffin will claim this "pales into insignificance" to the actions of British forces.
He will tell a press conference in London, organised by the Stop the War Coalition: "We detained individuals and carried out our own interrogation before handing them over to the US. We were under no illusion as to what awaited the individuals handed over by us.
"For the British Government to claim that they only became aware of the use of British territory this week is disingenuous."
Despite years of denials the Government disclosed that two flights had refuelled on the British Indian Ocean island territory of Diego Garcia in 2002.
Mr Miliband said that the two flights had each been carrying a single suspect and six years on one of the men is still being held by the US at Guantanamo Bay, while the other has been released.
He said he had been told by the Americans that neither individual had been held at a secret detention centre or subjected to waterboarding or other forms of torture.
For the "avoidance of doubt", Foreign Office officials are now drawing up a list of flights involving UK facilities about which concerns have been expressed.
