"There are approximately 106 flights per day between the
United Kingdom and the United States. Although numerous
arrests have been made in the U.K. to disrupt and neutralize this
threat, the Department of Homeland Security (DHS) is taking a
number of heightened protective measures to ensure the continued
safety and security of our international and domestic air
travel."
Source: From TSA documentation accompanying the
notification of raised security levels in the UK and the US, as a
result of new terrorist threats recently uncovered in the
UK.