The long reign of Toyota Motor Corp.’s Lexus atop the U.S. luxury car market is almost certain to come to an end this year, a victim of an aging lineup and production disruptions caused by the March 11 earthquake in Japan.

Shutdowns at Toyota plants in Japan, where all Lexus models except one, the RX 350 sport-utility, are made, threaten its already thin hopes of retaining its crown for the year, concedes Mark Templin, Lexus general manager in North America.

“It will be some time before we see where we stand,” said Mr. Templin. As of early April, he said: “We are right on our sales goals; we are right on our budget.”

In the first three months of the year, Lexus’s U.S. sales declined 4.4%, even though overall auto sales rose. Among luxury brands, Lexus slipped to No. 3 for the quarter, behind BMW and Mercedes-Benz.

Toyota’s Japanese plants are due to resume production next week for 10 days, but at about half their normal production level, the company said last week. The auto maker recently warned U.S. dealers there could be “significant” supply shortages in May, June and July, prime U.S. car-buying months.

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