Twenty transceivers are being installed for the trial

Transport for London (TfL) is installing 20 microwave transceivers for a trial in Southwark, south London, starting in December with 500 drivers.

The beacons detect electronic tags in the cars which then work out what time of day and how long they were in the charge zone, and bill accordingly.

Currently motorists are charged �8 regardless of how long they are in the zone or what time of day it is.

TfL said it would examine the results of the six-month experiment before deciding whether it should roll it out across the rest of the central London zone.

Environmental opportunity

But a spokesman said it was not likely until after 2009. He added a smaller trial involving just two beacons and about 12 drivers proved almost 100% accurate.

The more intelligent system could also provide an environment opportunity.

TfL's Michelle Dix told BBC News: "In the same way we have different charges for what we call alternative fuel vehicles, which are very clean and pay nothing, we could explore if different types of vehicles could pay different amounts depending on how much they pollute."

The congestion charge was introduced in February 2003 and has been credited with reducing traffic in the controlled zone by about 30%. Journey times have fallen by 14%.

From February 2007, the zone will be extended to include parts of west London.

The charge has faced criticism, especially from business groups which claim its has affected trade.