LONDON (Reuters) - A U.S. troop pullout from Iraq would leave the country as a potent launchpad for international terrorism and Washington would be forced to go back in within a couple of years, a leading al Qaeda expert said on Tuesday.

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."

Democrats are pushing for a time frame for withdrawal, something the White House opposes as sending the wrong message to U.S. forces, allies and enemies alike. Some Republicans are also questioning the war more publicly.

Britain, the leading U.S. partner in Iraq, is scaling back its troops there, although the idea of an immediate pullout was rejected this week by Gordon Brown, the man expected to succeed Bush's close ally Tony Blair as prime minister next month.

Addressing the same conference, a top British security official acknowledged home-grown Islamist militants had exploited the British troop presence in Afghanistan and Iraq for propaganda purposes.