The self-styled Caliph of Cologne, Metin Kaplan, heads the first organisation to be banned under new German anti-terror laws following 11 September.
Metin Kaplan has built up a following of 1,100 people across Germany
He has served a four-year prison sentence in Germany for inciting members of his group, the Caliphate State, to murder a rival Islamic leader.
A German court has now ruled that he can be extradited to Turkey to face treason charges.
The decision overturns a previous ruling that he was unlikely to receive a fair trial in Turkey.
His calls for the violent overthrow of the Turkish state and its replacement with an Islamic regime have made him a wanted man in Turkey.
Mr Kaplan has built up a following of 1,100 people across Germany, half of them in the western region of North Rhine-Westphalia.
Though the group has made recent attempts to distance itself from terrorism, it has not always been so.
According to intelligence reports, a group of his followers travelled to Turkey at the end of October 1998 to carry out two attacks.
Turkish officials say they planned to fly a plane into the mausoleum of Ataturk, the founder of modern Turkey, on the 75th anniversary of the country's establishment.
As that was taking place, militants were supposed to storm a mosque in Istanbul.
Turkish authorities say they foiled the plans, and 25 people were arrested.
Mr Kaplan has declared that it is every Muslim's duty to wage holy war.
"Every Muslim should overthrow the bad regimes in his country and set up an Islamic state," he has said.
Islam and democracy, he claims, are incompatible.
Mr Kaplan came to Germany in 1983, claiming asylum as he was then threatened with the death penalty in Turkey.
He took over the Association of Islamic Organisations and Communities on his father's death in 1995, winning out over his rival Ibrahim Sofu.
Mr Kaplan called for his opponent's death, and in 1997 Sofu was assassinated by three masked men.
In November 2000, Mr Kaplan was convicted for incitement to murder and sentenced to four years in jail.
Since then, intelligence reports say his followers have become even more devoted to Mr Kaplan, considering him a martyr for the cause of Allah.
All members of his organisation swear an oath of allegiance and are bound in unquestioning loyalty to him.
Mr Kaplan is believed to have a fortune worth millions.
Nonetheless, he claimed social benefits in Cologne for many years until 2m Deutschmarks (1m euros, £700,000) in cash was found in his flat.