The head of MI5 believes the terrorist threat to Britain has been reduced following a series of high-profile successes by the security services.

Jonathan Evans said that recent breakthroughs have forced terrorists to "keep their heads down". He pointed to 86 successful prosecutions in terror trials since January 2007, and the fact that in more than half the accused pleaded guilty.

"That has had a chilling effect," he added. "We have probably seen fewer 'late-stage' attack plans over the last 18 months."

But he believes that al-Qaeda is still determined to carry out major attacks in the UK and has the capacity to do so. "There is enough intelligence to show they have the intention to mount an attack here," he said.

He also warned that the global economic crisis was bringing potential new dangers for the country. He said: "Our focus in the next few years will be international terrorism, al-Qaeda and its associates, but we are also looking at the global economic crisis.

"Where there have been watershed moments, there have often been national security implications from that - a new alignment. We have to maintain flexibility and respond to threats. The world will not stay the same."

Mr Evans revealed that the terrorists who launched the Mumbai attacks in November had indirect links to Britain and stressed the importance of events in Pakistan and Afghanistan for the UK's security.

He said 75 per cent of MI5's investigations had connections with Pakistan.

The security chief also predicted that the Israeli invasion of Gaza would see "extremists try to radicalise individuals for their own purposes".

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