The London Congestion Zone is an area in the centre of London which if entered into in a vehicle anytime between 7am and 6pm (Monday to Friday), it will be necessary to pay the the congestion charge of 10 pounds for that day.
The congestion zone is monitored by cameras at every entry point which read your vehicle's number plate and through the DVLA can determine the vehicle's owner and their address.
Although there are signs informing you that you are about to enter the congestion zone, there are no toll barriers, and so it is up to you to remember to pay the charge on that day or you will subsequently receieve a letter informing you that you did not pay the charge and you will be subject to a large fine of 60 pounds.
Payment of the Congestion Charge can be made to Transport For London (TFL) either:
There is now a web application available for your iPhone or Blackberry which will show you how far at any point you are away from the congestion zone and will alert you if you enter the zone during a chargeable period. Further detail can be be seen at the London Congestion Zone checker web site
London Congestion Zone Checker Details on the Congestion Zone Checker About W A Bridge Ltd