TOKYO—Ratings firm Standard & Poor’s cut its outlook on quake-hit Japanese auto makersToyota Motor Corp., Nissan Motor Co. and Honda Motor Co., which saw their domestic production plummet by more than half in March from supply-chain disruptions.

The twin hits come a few days before Japanese car makers start reporting their latest earnings, in which they are expected to provide grim forecasts for the current fiscal year, which started this month.

The move by Standard & Poor’s Ratings Services indicates the increased likelihood of a downgrade for the auto makers as well as three suppliers the firm included in its assessment. Japan’s auto makers have struggled with parts shortages since last month’s disaster. A host of companies in the electronics, industrial and other sectors have face supply-chain problems in the wake of the catastrophe.

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