Cameron has announced he is convening Cobra in response to the Woolwich 'knife' attack - but what is Cobra?

Cabinet Office Briefing Room (Creative Commons)

In response to reports of a 'frenzied machete attack' in south-east London, David Cameron has asked Home Secretary, Theresa May to chair a Cobra meeting.

Cobra is an acronym for 'Cabinet Office Briefing Room', the room in which the UK Government convenes a committee in response to a national crisis. There are no set members of Cobra, but it is usually chaired by either the Prime Minister or another senior cabinet member, in this case, Home Secretary, Theresa May and includes other ministers as needed. In this case Defence Secretary Phillip Hammond, Met chief Bernard Hogan Howe and London Mayor Boris Johnson.

The US equivalent of Cobra is the White House Situation Room.