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The motoring organisation says, in some cases, drivers whose cars have failed the roadside tests have been issued with £60 penalty tickets - only to find their car passed the same check at an MoT testing station.

The RAC added that it was concerned that private cars and cabs were being targeted more than buses and taxis in the fight against pollution.

Roadside tests are being run in seven areas of the country, and they may be introduced nationwide.

The RAC said that allowing local authorities to conduct roadside emission tests is vital if pollution levels are to be reduced.

But it added that undercover works by RAC inspectors "had found evidence of inconsistencies between the MoT tests and roadside tests carried out by local authority enforcers".

Clean bill of health

The RAC gave examples of these alleged inconsistencies.

They included an RAC member in Wembley, north-west London, who failed a roadside check and was fined £60.

He immediately took his B-registration Volvo to an MoT testing station where it received a clean bill of health.

The car of a Surrey RAC member also failed an emissions test one week after a full service.

Four days later the car passed its MoT emissions check.

Glaring inconsistencies

RAC technical engineers also took a car to free emissions check at a supermarket.

The car failed, but passed an MoT test half an hour later and nine miles away.

"On-street emissions enforcement is vital, but pilot tests are showing glaring inconsistencies and problems with enforcement," said Edmund King, RAC head of campaigns.

He went on: "The longer term environmental objectives of roadside emissions enforcement will be put in jeopardy if public support is lost because of shoddy testing and an unfair fining regime.

"Ministers must carefully examine how local authorities are running emissions enforcement before they approve any national or permanent scheme."