Cairo (AFP) - The head of Egypt's Al-Azhar university, one of the world's leading Islamic seats of learning, has been replaced after labelling a controversial Muslim reformer an apostate, the institution said.

The development came as Al-Azhar is pressured by critics who say the venerable Sunni Muslim authority has not done enough to counter Islamist extremism.

Ahmed Hosni Taha, the acting university president, had been forced to apologise on Thursday after saying reformer Islam al-Behairy was an "apostate" for attacking some of the founding scholars of Islamic law.

His apology was followed by a statement on Friday from Al-Azhar saying that Grand Imam Ahmed al-Tayeb, who heads the institution that runs the university, had replaced Taha.

Taha had made the remarks about Behairy during a television interview.

"My response...was incorrect and it contradicts the way of Al-Azhar," Taha said in an apology posted on the university's website.

Behairy was a talk show host who had infuriated Al-Azhar's traditional clergy with attacks on canonical religious books and some of Sunni Islam's most important scholars.

He was sentenced to a year in prison for "insulting religion" and released in late 2016 in a presidential pardon.