Islamic State chief Abu Bakr al-Baghdadi has urged fanatics to free thousands of jihadis in a huge uprising.
Security experts fear terrorists, including dozens of Brits, could be sprung in a series of violent jail-breaks in Syria and Iraq.
The anti-IS coalition is operating on “borrowed time”, claims the US-based Institute for the Study of War.
The think-tank said: “Islamic State is preparing to free its fighters.
“Emir Abu Bakr al-Baghdadi signalled this campaign in a speech released on September 16, calling on fighters to liberate prisoners and save women in displacement camps.”
Kurdish Syrian Democratic Forces, who have led the struggle to smash IS, are battling to maintain security in refugee camps and prisons.
Brits in jail include Jihadi John’s friend Alexanda Kotey, ‘Jihadi Jack’ Letts and Hamza Parvez.
Jihadi brides, like former British national Shamima Begum, are in detention camps.
It is now feared IS fanatics could be smuggled to Europe to spread mayhem.
IS fighters went underground after the fall of its strongholds of Mosul, Iraq, and Raqqa, Syria.
They were told to form “sleeper cells” and hide weaponry.
The think-tank added: “IS is already raising funds via encrypted channels in displacement camps in Syria.
"It is likely organising similar activity in Iraq.”
Meanwhile, an island "infested" with ISIS fighters was annihilated by almost 40 tonnes of bombs dropped from US warplanes.
Dramatic aerial footage shows explosions as bombs hit Qanus Island followed by huge mushroom clouds billowing into the air over the Tigris River in Iraq.
The island had become a "safe haven" and "major transit hub" for the terrorists, who hid within thick vegetation, as they moved into Iraq from neighbouring Syria, the US-led coalition said.
But the hideout was obliterated and an unknown number of jihadists were killed as US Air Force F-15 Strike Eagle and F-35A Lightning II aircraft, alongside Iraqi warplanes, launched airstrikes this week.