David Cameron will appeal to international leaders to step up support for Somalia in an effort to stabilise the country described as "the world's most failed state".

Representatives of more than 50 countries are gathering in London on Thursday for a high level international conference amid fears the east African nation is becoming the new breeding ground for international terrorism.

Cameron said he wanted to "forge a new momentum" and put in place the "building blocks" to create a stronger and safer Somalia after two decades blighted by war, piracy, terrorism and famine.

Among the senior figures attending will be United Nations Secretary General Ban Ki-moon, US Secretary of State Hillary Clinton as well as the leaders of neighbouring African states.

Hopes for progress have risen in recent months after troops from the African Union force in the country (Amisom) succeeded in driving the Islamist al Shabaab group from the capital Mogadishu.

They were further bolstered by reports that the al Shabaab stronghold of Baidoa in the south west of the country has fallen to troops from neighbouring Ethiopia and Somali government forces.

The UN Security Council on Tuesday voted unanimously to increase the Amisom presence from 12,000 troops to 17,700 while increasing funding and expanding its area of operations.

Nevertheless al Shabaab remains in control of much of the country and earlier this month it underscored its commitment to violent extremism by publicly declaring its affiliation to al Qaida.

A recent report by the Royal United Services Institute thinktank estimated that there were currently around 200 foreign fighters in training camps in the country - with around a quarter of them coming from Britain.

The report echoed a warning made by MI5 director general Jonathan Evans in 2010 that it was "only a matter of time" before there were terrorist attacks on the streets of Britain inspired by those fighting in Somalia.

With the London Olympics later this year, Cameron has acknowledged that the security threat from al Shabaab was "real" and "substantial".

"The threat to our national security is growing. Young British minds are being poisoned by radicalism. Pirates are disrupting vital trade routes and kidnapping tourists and aid workers," he said in a statement ahead of the conference.

"The Shabab control a third of the country. Famine and instability in Somalia are spreading instability across the wider region.

"Our national interest is clear: we can't just sit back and let all this carry on."

Officials said Cameron's decision to summon the conference - being held at Lancaster House - reflected the deep concern in Whitehall at the threat posed by the continuing instability.

"It is clear that there are British nationals who have been among the foreign fighters who find a haven in al Shabaab areas," said one senior diplomat.

"We know al Shabaab has reach beyond Somalia. What we are talking about is not simply a threat to the region - it is a threat to the UK."

Downing Street has sought to play down reports that Britain and other EU nations have been looking at the possibility of airstrikes against al Shabaab bases.

"We have been focused on pursuing a different strategy to a military strategy," the Prime Minister's spokesman said.

However the spokesman also acknowledged that Britain had military "assets" in the region, including Royal Navy warships operating with the international anti-piracy task force.

British officials said they were looking for an "action-orientated" approach at the conference with agreement on a series of practical measures to tackle piracy and terrorism, stamp out corruption, strengthen the political process and bolster humanitarian support.

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