Abdullah Qardash has killed dozens of IS “traitors” as the deputy of al-Baghdadi, whose health is thought to be deteriorating after the terrorist leader was reportedly injured in fighting and last appeared in a video in April.
Security sources have said the move could spark a new wave of terrorism against the US and allies including Britain.
Qardash, like Al Baghdadi, both share a hate for the West after he was detained in Iraq by US forces that toppled Saddam Hussein in 2003.
“He has almost certainly overseen the torture and execution of a great many members of Islamic State”
A security source told the Mirror: “Qardash is extremely dangerous.
Al Baghdadi said in his last public message in April that he had launched a war of attrition ordained by God.
The announcement followed the defeat of the group by US-led Kurdish militia in eastern Syria and the killing of many of his lieutenants by US forces.
Al Baghdadi, who is in his late 40s, was sitting throughout the 18-minute video but otherwise did not show signs of ill health.
Unsubstantiated reports had described him as dead or wounded by US bombing, or living as a fugitive in Syria, Iraq and even Libya.