Judged on reliability, the two marques finished at the bottom of a 38-strong table of car manufacturers compiled by What Car? and Warranty Direct.
The table analysed thousands of cars that were three- to eight years old, assessing reliability based on a car’s failure rate, age, mileage and cost of repair.
What Car? said: “Luxury and prestige marques Bentley and Porsche are the most likely to break down.”
Top manufacturer for reliability was Honda, with Suzuki second, Toyota third and Chevrolet and Mazda sharing fourth place.
Overall, the most reliable models were the Honda Jazz and the Mitsubishi Lancer, both of which also had reasonable average repair costs.
The least reliable model was the Audi RS6, which was also the most costly to fix, with an average repair bill of £1,003.
Electrical faults were the most common across all cars, accounting for almost a quarter of visits to garages.
Axle and suspension faults were also common, affecting 22 per cent of cars.Air conditioning was the cause of least concern to motorists, being reported by just 3 per cent of owners.
What Car? editor Jim Holder said: “Honda’s success in the reliability index is chiefly down to low failure rates. But, when things do go wrong, the cars are also relatively cheap to fix.
“Reliability is always one of the key attributes buyers look for when considering a used car purchase, so manufacturers that consistently demonstrate durability will always do well with the consumer.”
The list price of Bentleys ranges from £136,250 for the 4.0 V8 Continental GT Convertible up to £224,700 for the Mulsanne 6.75 V8.
BEST AND WORST
1 Honda (Honda Jazz below)