The terrorist threat to the UK has been reduced after a series of successful prosecutions, MI5's chief has said.
Jonathan Evans said the 86 prosecutions in the last two years had had a "chilling effect on the enthusiasm of terror networks".
But he said al-Qaeda remained determined to launch attacks in the UK.
And he said the worldwide economic downturn could bring new threats to national security - as it would bring a "new alignment" in global affairs.
'Watershed moments'
He said there had been 86 successful prosecutions in terror trials since January 2007, and more than half of the accused had pleaded guilty.
"That has had a chilling effect. We have probably seen fewer 'late-stage' attack plans over the last 18 months," he said.
"We have to maintain flexibility and respond to threats. The world will not stay the same"But he added: "There is enough intelligence to show they [the terror networks] have the intention to mount an attack here.
"There is a significant number of individuals in active sympathy. They are doing things like fund-raising, helping people to travel to Afghanistan, Pakistan and Somalia."
Speaking on the centenary of the creation of MI5, Mr Evans said the security service would be looking at the global economic crisis.
"Where there have been watershed moments, there have often been national security implications from that - a new alignment," he said.
"We have to maintain flexibility and respond to threats. The world will not stay the same."
BBC security correspondent Gordon Corera said it was unprecedented for a director general of MI5 to give an interview.
Our correspondent added Mr Evans had said there was no single path to extremism, but foreign policy remained a factor.
"Mr Evans predicted the Israeli incursion into Gaza, as well as the conflict in Afghanistan would be used by extremists to try and radicalise individuals," he said.
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