The director general of Britain's top security agency has warned that the Iranian threat should not be underestimated.

MI5 chief Jonathan Evans told guests at the Lord Mayor of London's defence and security lecture that it was important to view Iran as part of the global terror threat.

He said: "We have seen in recent months a series of attempted terrorist plots against Israeli interests in India, Azerbaijan and elsewhere.

"So a return to state-sponsored terrorism by Iran or its associates, such as Hizbollah, cannot be ruled out as pressure on the Iranian leadership increases."

Mr Evans also revealed that his agency's assessment was that Britain "has experienced a credible terrorist attack plot about once a year since 9/11" and said that the London Olympics presented "an attractive target for our enemies".

But he said that he did not believe this country was an easy target for terrorist plotters.

In a speech that also covered the rise in the risk of cyber attacks, Mr Evans warned that British "would-be jihadis" were now choosing the Arab world as a region to train for terrorist activity.

"Some will return to the UK and pose a threat here. This is a new and worrying development," he said.