“A judgment by MI5 [U.K.’s counterintelligence and security agency] and the police that a Mumbai-style attack in the UK is now less likely led to a reduction on Monday [July 11, 2011] of the terrorist threat from ‘severe’ to ‘substantial,’ Whitehall sources have told the Guardian. …The threat level was increased to severe in January 2010 after the failed attempt by the so-called ‘underpants’ bomber, Umar Farouk Abdulmutallab, to bring down a Detroit-bound passenger aircraft. …The change in the official threat assessment–first made public in 2006—means that an attack in Britain is no longer classified as ‘highly likely’ but is instead regarded as ‘a strong possibility.’ Theresa May, the home secretary, said: ‘The change in the threat level to substantial does not mean the overall threat has gone away–there remains a real and serious threat against the United Kingdom.’ …The threat level was last at critical in June 2007, following an attack at Glasgow airport and failed car bombings in central London.”
– Richard Norton-Taylor and Alan Travis, “MI5 and Police Reduce UK Terror Threat From ‘Severe’ to ‘Substantial,'” The Guardian, July 11, 2011