Muslim religious teaching needs radical reform to stop the spread of extremism, according to the Grand Imam of Egypt's prestigious al-Azhar University.
Sheikh Ahmed al-Tayeb told a conference organised by the Muslim World League in Meccca and attended by senior clerics from across the Muslim world that extremism had flourished because of a misreading of the Koran.
Al-Tayeb said there had been "a historical accumulation of excessive trends" that had led some people to embrace a misguided form of Islam.
One of the factors which has rendered Islamic State's brand of ideology so toxic is its habit of pronouncing other Muslims as apostates. Al-Tayeb said: "The only hope for the Muslim nation to recover unity is to tackle in our schools and universities this tendency to accuse Muslims of being unbelievers."
He referred to "terrorist groups... who have opted for savage and barbaric practices" without naming Islamic State specifically.
He blamed a "new global colonialism allied to world Zionism" for unrest in the region, which he said had exploited "confessional tension" in Iraq, Syria, Yemen and Libya.
The conference heard from Saudi Arabia's King Salman, whose speech was read by the Governor of Mecca. The king described terrorism as "a scourge which is the product of extremist ideology" and as "a threat to our Muslim nation and to the entire world".
It is not the first time the Grand Imam has condemned extremist ideology; in a significant intervention in December he spoke against fundamentalist readings of the Koran which spawned jihadi ideology. However, while he and al-Azhar University have enormous prestige in the Muslim world, hardcore Islamic State ideologues would dismiss him as speaking for the modernist interpretations of Islam which they reject outright.