"Al-Qaeda is no longer a functioning organisation after being ripped apart by the Islamic State," the Guardian newspaper reported, quoting two of the terror group's most important spiritual leaders.
Abu Muhammad al-Maqdisi, regarded as the most influential jihadi scholar alive, told the paper that Zawahiri "operates solely based on the allegiance. There is no organisational structure. There is only communication channels and loyalty."
Maqdisi counts 63-year-old Zawahiri as a personal friend.
Zawahiri, who became al-Qaeda chief after the killing of Osama bin Laden by the US special forces in May 2011, carries a USD 25 million reward on his head.
Another radical preacher Abu Qatada, a Jordanian who was based in London before being deported in 2013, said Zawahiri is "isolated" and admitted that ISIS have been winning the propaganda and ground war against Al-Qaeda.
Qatada, described by the British government as a "truly dangerous individual", said ISIS members were extremists and a "cancer" growing within the jihadi movement following their assault on al-Qaeda over the last two years.
ISIS is an al-Qaeda splinter group and it has seized hundreds of square miles in Iraq and Syria.
Al-Qaeda has distanced itself from the group, chiding it for its lack of teamwork in its aggressive, brutal expansion.
ISIS leaders have described al-Qaeda as a "drowned entity" and declared that they will not tolerate any other jihadi group in territory where they are operating. The group led by Abu Bakr al-Baghdadi, a former Al-Qaeda commander in Iraq, has gone on to build a global network of affiliates and branches that now stretches from Afghanistan to West Africa.
The paper said that over the past year, a group of junior and mid-level analysts have concluded that ISIS advances have pushed al-Qaeda to the margins of global jihad.
The donations for Al-Qaeda, which once came in waves of "hundreds of thousands", have dried up as donors directed their money to ISIS, the paper said another report.
Citing a former al-Qaeda member, the paper said that one of his sources in Pakistan's tribal areas said the finances of Al-Qaeda central in Waziristan were so desperate that it was reduced at one point last year to selling its laptops and cars to buy food and pay rent.
Quoting a series of interviews with senior jihadi ideologues, the paper said that ISIS has successfully launched "a coup" against Al-Qaeda to destroy it from within.
"As a consequence, they now admit, Al-Qaeda -- as an idea and an organisation -- is now on the verge of collapse," it added.