Islamist terrorists have switched their focus to North Africa, leaving Britain facing a “generational struggle” in the region to combat their “poisonous ideology”, David Cameron said today as he signalled a dramatic shift in foreign and defence policy.

In a Commons statement on the Algerian hostage crisis, he argued that al-Qaida allies operating across a vast tract of lawless territory had to be taken on and defeated to prevent them using it as a base to mount attacks around the world.

Mr Cameron announced Britain would join the hunt for the terror chiefs behind the raid on the In Amenas gas facility and promised extra support for French-led military action against insurgents in northern Mali.

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Echoing language used by Tony Blair after the 9/11 attacks on the United States, he vowed not just to contain the extremists, but to overcome them.

He told MPs: “This is the work our generation faces and we must demonstrate the same resolve and sense of purpose as previous generations have with the challenges that they faced in this House and in this country.”

The National Security Council (NSC) – which comprises senior ministers, the heads of MI5, MI6 and GCHQ and the Chief of the Defence Staff – will meet today to begin plotting Britain’s response to the terrorist threat across the North Africa as well as Yemen in the Arabian peninsula.

Ministers believe that defeating militants in Mali is crucial to stopping them spread their influence to neighbouring Niger and Mauritania, as well as remote parts of Algeria and Libya. Their biggest fear is that Nigeria – where the Boko Haram jihadist group is active in the north of the country – could become destabilised.

Separately African forces have spent months battling fighters from the al-Shabaab group in Somalia.

Mr Cameron told MPs that the Algerian attack highlighted a shift in the danger posed by Islamist terrorists globally.

Over the last four years the proportion of terrorist plots against Britain originating in Afghanistan and Pakistan had fallen from three-quarters to less than half.

But he added: “At the same time al-Qa’ida franchises have grown in Yemen, Somalia and parts of North Africa.”

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And he warned the region was “becoming a magnet for jihadists from other countries who share this poisonous ideology”, pointing to reports of non-Algerians taking part in last week’s attack.

The Prime Minister said: “In sum, we must frustrate the terrorists with our security, beat them militarily, address the poisonous narrative they feed on, close down the ungoverned space in which they thrive and deal with the grievances they use to garner support.”

The NSC meeting will consider bolstering transport support for French forces in Mali, where Britain has already lent two military aircraft, as well as the deployment of surveillance drones to the country. Britain will send a small number of troops – thought to be around 20 – to join a European Union mission to train the Malian army.

Mr Cameron also promised to commit “intelligence and counter-terrorism assets” to the manhunt for the terror network behind the atrocity, in which six Britons are feared to have died.

He did not elaborate, but the action is likely to involve special forces and undercover agents attempting to locate figures such as Mokhtar Belmokhtar, the alleged mastermind of the attack.

The Prime Minister added that he would place the issue of terrorism “right at the top of the agenda” for Britain's presidency of the G8 group of the largest economies in 2013.