Delivery companies want to be exempt from the charge

A survey by the Freight Transport Association (FTA) found firms were paying the charge but were still being stuck in traffic.

Operators said although queues are reduced inside the charging zone, they are worse outside.

The FTA has been campaigning for delivery vehicles to be excluded from the charge.

Tuesday's survey, which covered the operators of 6,000 vehicles, suggested the cost to the freight industry was running into millions of pounds.

Companies said, in the first month, the charge itself added up to �3.5m for delivery vehicles.

If the mayor is to continue to charge industry then he should ensure that a value-for-money improvement in operating conditions is provided Sue Moody, Freight Transport Association

On top of that were the costs of establishing a payment fleet scheme and the daily administrative costs of paying the charge.

Many companies complained of glitches in the administration of the scheme, which is run by Transport for London (TfL).

One found that on three occasions they had been charged twice for the same vehicle on the same day.

Others complained of delays in registering vehicles and IT problems.

The FTA's Sue Moody said: "If the mayor is to continue to charge industry then he should ensure that a value-for-money improvement in operating conditions is provided. So far he has not."

Since 17 February, vehicles in the central zone have been subject to a �5-a-day charge.

Exemptions are made for various groups, such as taxis, coaches, motor cycles and emergency vehicles.

But the FTA says delivery vehicles have no choice but to operate in the zone and the charge is an unavoidable stealth tax.