CATALYTIC converters may not be as effective at reducing emissions as was at first hoped, according to the Vehicle Inspectorate. The agency's annual report says that during the last year catalyst-equipped cars failed the MoT emissions test at a rate of 4.5% - a figure which it expects to rise as such vehicles get older.

'This is a surprisingly high figure since the limit values to be used were specified by manufacturers /importers and the vehicles were all under four years old when tested and therefore likely to be subject to regular maintenance in franchised dealers,' said the report.

'The evidence suggests that catalyst performance may not be so reliable as might have been expected when legislation was introduced. However, even where emissions have deteriorated beyond MoT limits, in most cases catalyst-equipped cars should still emit much lower levels of pollutants than older conventional petrol cars.'