Tariq Ramadan, the leading Muslim academic, has resigned his professorship at an American university after authorities refused to give him a visa.
Swiss-born Prof Ramadan is one of the most respected philosophers of religion and conflict resolution; he has argued for a more moderate and modern Islam, and was named by Time magazine as one of the world's top 100 influential thinkers this year.
But in July his American visa was revoked under the Patriot Act, adopted after the terrorist attacks on September 11, prohibiting him from taking up the post at the University of Notre dame in Indiana. They have so far refused to issue a new visa.
Today he announced his resignation of two professorships at the university - professor of Islamic studies in the classics department and professor of religion, conflict, and peace building - and accused the American authorities of attacking academic freedom.
In a written statement Prof Ramadan said: "No matter what decision I have taken today, I am still waiting for the American administration to reveal the results of their investigation so that my name can be cleared of all the untrue and humiliating accusations against me during these last few months. As yet, not a single piece of evidence has been produced to substantiate the claims made against me, which I believe is a classic case of infringement of academic freedom.
"My hope of teaching in the United States was based on the sincere aspiration to participate in the pressing and topical debates of our time. An unjust decision does not bar me from continuing this imperative struggle for dialogue and understanding between women and men, between religions and cultures."
He thanked the university and his supporters from around the world, and added: "It is they who are promoting pluralism and democratic debate. The American administration, on the other hand, seems to be unfortunately demonstrating signs of a rapid descent into a closed and worrisome unilaterism."
Prof. Ramadan claims he has been called an anti-semite and has been accused of having links to extremist Islamic groups.
The university expressed regret at the news. R Scott Appleby, director of the Joan B Kroc Institute for International Peace Studies, said: "We are disappointed that Professor Ramadan will not be joining our faculty. Faculty and students at Notre Dame and at other US universities were looking forward to engaging him productively on a variety of issues central to our times. Such dialogue, we believe, is an essential requirement to a deeper understanding of the complexity of the Muslim world."
In November, speaking at a debate organised by the Guardian, he argued that the British Muslim community was ahead of any other Muslim community in Europe and has a far more sophisticated understanding of its place in society, but, he added Muslims should, "get rid of the defensive attitude".