Parked inconspicuously on street corners, a squad of unmarked white vans spends each day scrutinising London's traffic in search of "vosis" - vehicles of special interest.
Operated by the out-sourcing firm Capita, each van has a camera perched on top which registers the number plate of each passing car.
In a cramped space in the back, two staff monitor a computer which sounds an alarm every time a persistent offender goes by.
The vans are not popular with Londoners. A suggestion of painting a big red "C" on the side of each was vetoed, on the grounds that employees could become targets.
One staff member said: "Someone once chucked a sandwich on to the roof. And we once had an egg thrown at one of our vans."
Transport for London is using intelligence-led techniques to enforce its £5-a-day congestion charge. Vehicles which routinely fail to pay the fee are entered on a database of "vosis".
By registering every sighting of a "vosi", TfL builds up a pattern of the driver's habits, enabling the vehicle to be tracked down and impounded.
The vans are TfL's frontline troops in a battle against criminals who "clone" the number plates of innocent motorists in an attempt to avoid speeding cameras, registration fees and congestion charges.
Sophisticated fraudsters make sure they copy a number plate from a car of the same model and colour.
Enforcement has been among the trickiest challenges for TfL's congestion charging team. TfL has lost half of the appeals brought against penalty charge notices. There have been persistent problems with drivers incorrectly entering registration details.
The number of penalty charge notices sent daily has settled at 4,000 - and many of these are likely to be to unlicensed drivers, who persistently flout the charge in the expectation that they cannot be traced.
The London mayor, Ken Livingstone, has pledged to tow away, and ultimately crush, vehicles which do not conform to the rules.
But he has yet to do any towing: TfL has introduced an extra check, sending out a "last chance" letter asking motorists to get in touch even after three "final" warnings.
Jonathan Simpson, spokesman for the RAC Foundation, said: "We think there have been significant problems.
"There have been motorists as far afield as Aberdeen accused of driving into the zone."
Even foreign vehicles were supposed to be liable. Mr Livingstone signed up an international debt collection agency, Intrum Justitia, to chase overseas drivers who refuse to pay. But in many countries, including Germany and Denmark, local law prevents the agency from tracing owners of cars.
