THE last 18 months of upheavals in the Arab world have spawned a new generation of British-born terrorists after al-Qaeda moved into unstable countries and began training potential bombers, Britain's domestic intelligence chief has warned.

Jonathan Evans, the head of MI5, said the terrorist network had taken advantage of the revolutions that began last year to spread its influence.

British militants were known to be receiving training in countries such as Libya and Egypt, mirroring what has happened in Yemen and Somalia, he claimed.

Speaking in the inaugural Lord Mayor's Annual Defence and Security Lecture in London, Mr Evans warned that it would be ''extraordinary and self-defeating'' if British government proposals to give the security services greater powers to snoop on emails and phone calls were blocked.

He said that a new terrorist threat had emerged from an ''Arab world in radical transition'' and that while events offered hope in the long term, there was a ''more immediate problem'' as al-Qaeda returned home. ''This is the completion of a cycle: al-Qaeda first moved to Afghanistan in the 1990s due to pressure in their Arab countries of origin,'' Mr Evans said. ''They moved on to Pakistan after the fall of the Taliban.

''And now some are heading home to the Arab world again. 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.''

Mr Evans said it was a sign of al-Qaeda's change of focus that a few years ago, three-quarters of MI5's work centred on Afghanistan and Pakistan, whereas less than half did now.

''Al-Qaeda affiliates in Yemen, Somalia and the Sahel have become more dangerous as al-Qaeda in Pakistan has declined and we see increasing levels of co-operation between al-Qaeda groups in various parts of the world,'' said Mr Evans.

The network was also active in Syria and in West Africa, he warned.

Suggestions that al-Qaeda's threat had ''evaporated'' following the death of Osama bin Laden were wide of the mark, he said, adding that Britain had ''experienced a credible terrorist attack plot about once a year since 9/11''.

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