Maajid Nawaz, one of the most senior members of the radical Islamist party Hizb-ut-Tahrir, talks exclusively to Newsnight on BBC Two tonight.
On the programme to be broadcast at 10.30pm Nawaz reveals how Hizb-ut-Tahrir advocate the killing of millions of people to unite and expand an Islamic super-state and why he resigned from the party.
For 12 years Maajid Nawaz was inside Hizb-ut-Tahrir, not only propagating their views in Britain, but exporting them to Pakistan and Denmark. He was imprisoned in Egypt for four years for being a member of the party. Up until May this year he was on their leadership committee.
The establishment of the Khilafah – an Islamic state across the Muslim world under Sharia law – is the central aim of Hizb-ut-Tahrir. Last month they held a series of international conferences, the largest in Indonesia, to "accelerate" its establishment.
Hizb-ut-Tahrir publicly state that this would be achieved "without resorting to violence" and "following an exclusively political method".
But Maajid Nawaz has told Newsnight that, once that state is established, the party does advocate violence, and violent expansion beyond the Muslim world.
He says: "They are prepared to, once they've established the state, to fight other countries and to kill people in the pursuit of unifying this state into one state. And what I'd like to emphasize is that such a policy is not agreed upon within Islamic theology.
"... Hizb-ut-Tahrir privately and publicly condemn terrorism but the point I'm making is that's not the danger I'm concerned about.
"The danger I'm concerned about is creating a mentality, a psyche that can allow a state and it deems it acceptable for a state en masse to kill people in the cause of an ideology."
Hizb-ut-Tahrir is a global Islamist movement founded in 1953, committed to the establishment of a unitary Islamic state across the Muslim world, under Sharia law. It is banned in many Middle Eastern countries, including Egypt, Syria and Turkey.
After the July 7th bombings the then Prime Minister Tony Blair moved to ban the organisation in Britain but there was insufficient evidence to do so.
The debate over proscribing the party has centred on how extreme the movement is and its stance on violence. It insists that it works through exclusively political means.
Nawaz says that his time spent in prison meant he started questioning if there was a better way "than just meeting oppression and anger with more anger and more oppression".
He developed serious doubts, leading to a decision to leave.
"I regret my whole association with Hizb ut-Tahrir and the way in which I propagated those ideas.
"... I think that what I taught has not only damaged British society and British Muslim relations and damaged the position of Muslims in this society as British citizens, I think it's damaged the world."
Despite his criticisms of the party, Maajid Nawaz is not joining calls for Hizb-ut-Tahrir to be banned in Britain.
He says: " I expressly and explicitly say to the members I want them to leave Hizb ut-Tahrir because I believe Hizb ut-Tahrir is an obstacle to the Muslim community moving forward, not only in this country but in the world in general. And that's why I'm here, because I regret me being a part of that obstacle.
"My ideal scenario would be not to ban the party but it would be that through the power of discussion and persuasion and the strength of challenging thought with thought, that eventually the party would fizzle out in this country and hopefully generally throughout the world."