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.

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 Monday 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."

Not so long ago, 75 percent of the terror threats prioritized by MI5 had links to Afghanistan or Pakistan. But Britain's efforts, along with those of its international partners, has brought that percentage down to below 50 percent, Evans said.

"You could say that we are near to reaching a form of stalemate," said Evans. "They haven't stopped trying but we have got better at stopping them."

But the terrorist threat is also widening to include al-Qaida affiliates in Mali, Yemen, Somalia and parts of the Middle East, he noted.

Although the Arab Spring revolutions have brought about radical political changes in some countries, they have also brought fresh opportunities for al-Qaida affiliates to seek refuge.

"Some are heading home to the Arab world again," Evans said. "And a small number of British would-be jihadis are also making their way to Arab countries to seek training and opportunities for militant activity, as they do in Somalia and Yemen. Some will return to the UK and pose a threat here. This is a new and worrying development."

MI5 has grown since the 2001 terror attacks in the United States, going from 1,800 to 3,800 staff -- some of whom joined after Britain's own homegrown suicide bombings that killed 52 people in 2005.

Since then, several international terror plots have been hatched in the United Kingdom, including the 2006 trans-Atlantic airliner plot to down several planes using liquid explosives. A handful of terror trials are also underway.

Evans stressed that other countries, such as Iran, shouldn't be underestimated in the evolving terror threat landscape.

"We have seen in recent months a series of attempted terrorist plots against Israeli interests in India, Azerbaijan and elsewhere," he said. "So a return to state-sponsored terrorism by Iran or its associates, such as Hezbollah, cannot be ruled out as pressure on the Iranian leadership increases."

In Tehran, Iran's foreign ministry warned Britain of "excuse-seeking" over the case.

"Countries should behave based on international measures. Their discriminative attitude will face our harsh and serious reactions," Iran's Foreign Ministry spokesman Ramin Mehmanparst told reporters when he was asked about remarks by MI5 chief.

Evans also said the threat of cyber security had recently become more prominent.

In one recent case, a major London-listed company incurred revenue losses of 800 million pounds ($1.2 billion) because of a cyber attack, Evans said. He did not elaborate on the company's name or what country was behind the attack.

"What is at stake is not just our government secrets but also the safety and security of our infrastructure, the intellectual property that underpins our future prosperity and the commercially sensitive information that is the lifeblood of our companies and corporations," he said.

Evans has been in charge of MI5 since 2007.