Environment Correspondent Robert Pigott: "Testing has helped reduce pollution levels"

The study by the National Audit Office (NAO) - which investigates how efficiently government money is being spent - says tens of thousands of cars are sent back on the road despite their exhaust emissions being over the legal limit.

The report found the standard of testing in garages is often low, roadside tests can produce inconsistent results and it can be difficult to get access to exhaust systems on lorries.

A check on the exhaust has been part of the annual MoT roadworthiness test for the last five years. And the auditor general, Sir John Bourn, reported to Parliament that this has helped to reduce emissions of carbon monoxide. But more than 10% of vehicles still cause half the pollution on Britain's roads.

Spot checks 'unfair'

Eight local authorities are trying out a scheme whereby testers can impose on-the-spot fines at the roadside.

But motoring organisations say these are not fair because the MoT test differs from the roadside check and a driver who passes an MoT in the morning, can fail a roadside test in the afternoon.

Eight councils make roadside checks

"We've got to get this sorted out. You can't give someone an on-the-spot fine if they don't know they've got a problem," said Edmund King of the Royal Automobile Club.

The Automobile Association (AA) complained that the NAO report had confirmed its fears that drivers are not treated fairly.

AA policy manager Bert Morris said: "We warned the government before the trials started that there was a risk drivers could be penalised unfairly because the system worked against natural justice.

"Responsible drivers who do their best to properly maintain their vehicle are treated in exactly the same way as irresponsible drivers who do not. We must tackle grossly polluting vehicles, but to use a system which is so badly flawed is counter productive."

Parts warning

Testing does seem to have worked to an extent. In the year after it was introduced in 1994, there was a 14% reduction in emissions. Since then a lot of the dirtier vehicles have been weeded out so testing has had less of an impact.

The government's Vehicle Inspectorate is now considering ways to make the test easier for garages to apply, and from September 1999 all heavy goods vehicles will be tested with a meter.