Britain has seen a credible terrorist attack plot about once a year since the Sept.11 attacks — a worrying pattern as security officials brace for an array of threats ahead of next month's summer Olympics, the head of the country's domestic spy agency says, according to CBS News.

Although Britain's threat level is a notch below what it has been for much of the past decade, it is still at substantial. The level means an attack is a strong possibility.

"Our assessment is that Britain has experienced a credible terrorist attack plot about once a year since 9/11," MI5's Director General Jonathan Evans said in a rare public speech to the Lord Mayor's annual defense and security lecture in London.

"The (Olympic) games present an attractive target for our enemies and they will be at the center of the world's attention in a month or so," he said. "But the games are not an easy target, and the fact that we have disrupted multiple terrorist plots here and abroad in recent years demonstrates that the UK as a whole is not an easy target for terrorism."