A British man convicted of being a member of al-Qaeda has accused MI5 and police of attempting to pervert the course of justice.
Rangzieb Ahmed, from Rochdale, said he was offered a shorter jail term if he would drop claims UK officials were involved in his alleged torture.
Ahmed was jailed for life in December for directing terrorism. In the trial, Ahmed said he was tortured in Pakistan.
Whitehall said MI5 agents always operate within the law.
The North West Counter-Terrorism Unit denied any involvement in torture.
Ahmed accused the British of being complicit in his alleged mistreatment and he is now intending to sue.
In a letter seen by the BBC, his lawyers have written to the Crown Prosecution Service complaining about the alleged prison term reduction offer.
His solicitor has asked that all records of the visit - including any secret recordings - should be kept.
Ahmed became the first al-Qaeda suspect convicted in the UK of directing terrorism following a trial at Manchester Crown Court.
He travelled to Dubai from Pakistan via China en route to South Africa in December 2005 as part of a "major activity", prosecutors at his trial said.
Ch Supt Tony Porter, head of the North West Counter-Terrorism Unit, said it did not "participate in, solicit, encourage or condone the use of torture or inhumane or degrading treatment".
He added: "The case of Rangzieb Ahmed has been heard in court.
"The judges considered the allegations of UK complicity in mistreatment and made clear there was no evidence for this. These judgements are a matter of public record."