Rohan Gunaratna told a security conference at Lloyd's of London insurance market that Iraq, like Afghanistan in the 1990s, would become a "terrorist Disneyland" where al Qaeda could build up its strength unchallenged.
If U.S., British and other coalition troops withdrew from Iraq in the next year, he said,
"certainly the scale of attacks that would be mounted inside Iraq, and using Iraq as a launching pad to strike other Western countries -- countries in Europe, North America - would become such that after two or three years, the U.S. forces will have to go back to Iraq."
The Singapore-based academic and writer said the epicenter of international terrorism had already switched from Afghanistan to Iraq.
"In many ways, the terrorist threat has now shifted 1,500 miles closer to Europe."
A former head of Britain's foreign intelligence service MI6 described Gunaratna's analysis as convincing.
"Clearly al Qaeda are focusing on Iraq now, and focusing on some sort of propaganda victory over the United States,"Sir Richard Dearlove told reporters.
"Whether that's an actual victory or not, if they can claim in the Muslim world that they've done well, then that puts us in a very difficult position. This is really an aspect of withdrawal that hasn't been properly considered. That's why I think we can't just let Iraq go its own way."