Great Britain has raised its terrorism threat to "severe," the second highest level.

"The Home Secretary confirmed that the Joint Terrorism Analysis Center has increased the threat level in the United Kingdom from substantial to severe," British Prime Minister David Cameron told reporters, adding that severe means an attack is highly likely.

"This is the first time in three years that the threat to our country has been at this level."

The threat increase came as the crisis in Iraq and Syria worsens with Islamic State militants continuing to destabilize the region.

"With ISIL, we're facing a terrorist organization not being hosted in a country, but actually seeking to establish then violently expand its own terrorist state," he said.

"We could be facing a terrorist state on the shores of the Mediterranean and bordering a NATO member. Now we cannot appease this ideology, we have to confront it at home and abroad."

Cameron said he planned to introduce new legislation to make it harder for Britons to travel to Syria and Iraq to fight alongside Islamist extremists.