Cars made by Mercedes-Benz, BMW and Peugeot are using up to 50 per cent more fuel than their official laboratory tests suggest, a report claims.

Cars on average are consuming 40 per cent more fuel in 'real-life' conditions than in the figures put out by manufacturers, according to the report.

This 'gap' has increased from 8 per cent in 2001 and 31 per cent in 2012, but for some car-makers the discrepancy is higher than 50 per cent. This amounts to more than £350 a year in extra fuel costs for the average car.

The Volkswagen scandal involved the German car-maker inserting special software to rig the results

Heavy car users, particularly those using the larger engines favoured by Mercedes-Benz and BMW, could be spending thousands of pounds more a year on fuel.

The report, by environmental group Transport and Environment (T&E), is based on data collated by the International Council for Clean Transportation (ICCT), whose research exposed Volkswagen cheating over emissions tests centred on NOx, the harmful polluting gases in diesel..

There is no evidence that miles per gallon figures have also been manipulated in such a flagrant manner, but the report points out several tricks that could be carried out in lab conditions to lower the statistics.

European figures are based on so-called 'rolling road' conditions in an indoor lab, rather than the car being driven on an outside road.

Further measures such as using smaller wheels, over-inflating tyres, taping over gaps to reduce aerodynamic drag and taking off windscreen wipers would all help to lower the official fuel consumption figures in the laboratory, when compared to real conditions in the outside world.

The apparently more accurate figures used in the T&E report were based on both average fuel consumption figures collated from German drivers in real-life conditions, and carbon dioxide (CO2) emissions on roads rather than in labs. The latter is significant because CO2 emissions have a close correlation with fuel consumption.

The Mercedes E Class, A Class and C Class vehicles all give CO2 and mpg readings in 'real world' tests that are more than 50 per cent higher than those achieved in the laboratory. Hard on their heels are the BMW 5 series (49 per cent higher), Peugeot 308 hatchback (48 per cent), Renault Megane (45 per cent) and Volkswagen Golf (41 per cent).

The report says: 'The gap between official and real-world performance found in many car models has grown so wide that it cannot be explained through known factors including test manipulations.

'While this does not constitute proof of 'defeat devices' being used to fiddle fuel economy tests, similar to that used by Volkswagen, EU governments must extend probes into defeat devices to CO2 tests and petrol cars too.'

Greg Archer, clean vehicles manager at T&E, said: 'Like the air pollution test, the European system of testing cars to measure fuel economy and CO2 emissions is utterly discredited.

'The Volkswagen scandal was just the tip of the iceberg and what lies beneath is widespread abuse by car-makers of testing rules enabling cars to swallow more than 50 per cent more fuel than is claimed.'

The data in the T&E report is largely based on figures placed in an online questionnaire by drivers. In tests conducted by consumer magazine Which?, the average difference between the claimed and measured fuel economy figure was a more modest 12 per cent.