A couple weeks ago I had read about the new threat level system, but I hadn’t really paid too much attention. That said, it looks like the terror plot announced last night by the Home Office has moved London up to the highest level.
The threat levels are:
Critical – an attack is expected imminently
Severe – an attack is highly likely
Substantial – an attack is a strong possibility
Moderate – an attack is possible but not likely
Low – an attack is unlikely
So serious is the threat, that according to our corporate travel folks:

London Heathrow airport is now closed to all incoming flights that are not already in the air until further notice. Passengers due to travel today have been advised not to travel today unless absolutely necessary.
In addition, several airlines are cancelling flights, including British Airways, Lufthansa, Iberia, Air France and KLM.

Goodness, maybe its a good thing I didn’t fly off to some exotic local, to celebrate my upcoming birthday. Had I gone, I’d more than likely be flying around this time period.