If claim is true, London is under more menace than we thought

The claim by Yemen that Umar Farouk Abdulmuttalab was recruited by al Qaeda in London will heighten concerns about extremist activity in the capital.

What is known is that the Nigerian student began to dabble in radical Islamism while studying at University College between 2005 and 2008 and, say security sources, tried to make contact with known extremists.

While leading the university's Islamic Society, he also organised an event in 2007 involving several radicals, and invited speakers from a group which has promoted the views of Anwar al-Awlaki, the al Qaeda cleric, whom he is believed to have met in Yemen.

Both the Home Secretary Alan Johnson and senior Whitehall officials with knowledge of Abdulmuttalab's contacts in Britain insist that, so far, there is nothing to indicate that these links had brought him into direct contact with al Qaeda. Nor, they argue, is there any evidence that while in London he had embarked upon the allegedly violent path which has now led to his detention in the US.

This official confidence is reflected in the fact that the official threat level remains at "substantial" - two notches below the maximum "critical".

There is still a danger, however, that today's Yemeni claim might turn out to be correct. Until this investigation is completed the possibility remains that the scale of the al Qaeda threat to London could be more menacing than we know.