Macer, not his real name, told The Sun: "If the Syria civil war continues and the Islamic State rises up, we'll see a huge burst in terror attacks around the world.
He claimed that the HTS and Free Syrian Army (FSA), a group which helped the rebels topple Assad, had previously helped release ISIS jihadists from camps and "allowed them to go missing".
He warns that the civil war could continue for years - allowing ISIS to rise up and break free the 100,000 battle-hardened jihadists and their families languishing in Syrian camps amid the chaos.
The banker-turned-fighter fears the terrorists could launch a new wave of deadly terror attacks in Britain and across the world in a matter of months if ISIS is allowed to gain force and escape the camps.
Macer warned: "If al-Jolani takes control of the camps, if this new government in Damascus performs in the future as they have done in the past, then there's a real risk that they'll let ISIS prisoners go.
