Audi and BMW have been forced to defend their products after slipping down among the poorest performers in terms of car reliability, according to the latest findings published by consumer magazine Which?.

The two manufacturers have fallen to the lowest level of customer satisfaction in this year’s survey in respect of faults and breakdowns.

Volkswagen also failed to improve on its poor result in last year’s study. For the second year in succession the Audi TT fared the worst in the watchdog’s examination of breakdowns among cars aged up to two years. Plagued by ignition coil problems, on average one in five TTs suffers a breakdown, says Which?.

Audi argues it is close to beginning an upturn. In a statement, the manufacturer says the coil failure has been caused by an externally-produced part, which was rectified for production cars during 2003.

“We’re confident that our reliability rating will quickly return to normal now that our replacement programme is nearing completion,” it adds. “The Audi brand reliability level normally falls very close to, or better than, the average of the most recent Which? report.”

BMW describes the report as “concerning” but questions the accuracy of the watchdog’s study sample which, says BMW, includes less than 1% of all BMWs sold in the UK over the past two years.

“The sample just isn’t representative,” its spokesman maintains.

Which? stands by the results.