Toyota and its luxury Lexus brand slipped for the second consecutive year in an influential vehicle dependability study released on Thursday.

Toyota Motor Corp's <7203.T> Lexus fell to fourth in the J.D. Power and Associates Vehicle Dependability Study. Lexus had been No. 3 in the 2009 key quality ratings, losing the top spot for the first time since the once-dominant luxury line became part of the closely watched study.

The results come at a critical time for the world's top automaker, which is struggling to regain consumer trust after a damaging series of recalls that has tarnished its once-sterling reputation for quality and reliability.

Porsche, Ford Motor Co's Lincoln and General Motors Co's Buick won the top three rankings in the study, which tallies vehicle dependability based on problems reported in the first three years of ownership. Ford's Mercury nameplate ranked fifth, behind Lexus.

The Toyota brand slipped two notches from 2009 to the sixth spot, but still was the highest-ranking mainstream brand with four models placed among the most dependable in their segments, including the top-selling Prius hybrid.

Honda Motor Co's <7267.T> mainstream Honda line and the blue oval Ford nameplate followed Toyota in the list.

The J.D. Power rankings are closely watched by automakers since they are seen as a key indicator of the resale value of vehicles and customer loyalty.

The study is based on responses from more than 52,000 owners of 2007 model-year vehicles and was fielded between October and December. Toyota launched the first of its recalls over unintended acceleration in late November.

Ford, the only major U.S. automaker not to restructure under bankruptcy in 2009, outperformed rivals with all of its Ford, Lincoln and Mercury brands placed in the top 10 of the list.

Lincoln showed the most significant improvement in quality, climbing six positions from 2009.

The study also showed that seven of the 10 models with the lowest incidence of problems in the industry are from GM and Ford.

GM's Cadillac DTS luxury sedan had the fewest problems, with 76 issues per 100 vehicles, compared with the industry average of 155 problems. That marked the first time in more than a decade that a domestic model has scored the least number of problems in the study.

J.D. Power said that for several brands including Cadillac, Ford, Hyundai <005380.KS>, Lincoln and Mercury, consumer perceptions have not kept pace with their actual performance.

It takes considerable time to positively change consumer perceptions of quality and dependability -- sometimes a decade or more -- so it is vital for manufacturers to continually improve quality and also to convince consumers of these gains, said David Sargeant, vice president of research at J.D. Power.

Chrysler LLC, which emerged from bankruptcy in June under the management of Fiat , saw all of its Chrysler, Dodge and Jeep brands ranked below average in the study.

Chrysler was placed 20th, Dodge 28th and Jeep 34th in the study, which ranks 37 major nameplates.

(Reporting by Soyoung Kim, editing by Maureen Bavdek)