They have been pelted with eggs, bombarded with sandwiches and faced taunts from London's volatile commuters. But a fleet of vans enforcing London's congestion charge has met its match in Westminster council's traffic wardens.

Eleven congestion charging vans have been equipped with hi-tech cameras to catch those drivers who seek to evade the £5-a-day fee to enter the central area of the capital.

Westminster's Conservative-controlled council adamantly opposes the congestion charge, and is threatening to clamp the vans unless the Transport for London authority pays it up to £40,000 a year in parking charges.

A TfL insider said that Westminster was the only central London borough refusing to negotiate over parking fees for the vans, which typically spend an entire day in one location.

The borough has banned TfL from parking on yellow lines or in residents' bays, forcing the vans to park on meters costing £40 a day for each van.

A spokeswoman for the borough said: "Parking in Westminster is a service under huge demand - we cover much of central London."

She said Westminster would not budge on parking charges unless TfL, in turn, reconsidered its approach to vans delivering meals to schools in the borough, which must pay the congestion charge.

Capita, the firm that operates the vans for TfL, has trained its staff to deal with abuse from motorists. The company successfully blocked plans to paint a big red "C" on the side of each vehicle, fearing it would make them the focus of frustration for commuters.

Each van has an alarm which sounds whenever its cameras spot a serial non-payer - known as a "vehicle of special interest".

Seven weeks on from the introduction of congestion charging, TfL says its traffic counters across London show a 20% reduction in the number of vehicles on the roads.

The RAC recently concurred, saying traffic was down 15% during rush hours and 25% at off-peak times.

The scheme looks likely to be widely copied. In a study released by Deloitte Consulting yesterday, 32% of European city authorities said London's scheme had encouraged them to investigate the possibility of introducing similar charges.