Toyota, Honda, and Nissan have been placed on credit watch with what is termed negative implications by Standard & Poors, the New York ratings agency.
The move comes less than a week after Toyota - the third-biggest car maker in the world - warned it may lose money this year and Honda reported a 55 per cent profits slump. Last month Nissan decided to close a factory near Tokyo after losing Y108bn (pounds 692m) in the year to March.
Further evidence of the pressure on Japanese automotive exports was also provided yesterday by figures for July, showing a 13.9 per cent decline in vehicle shipments from a year earlier to 404,178.
Standard said its rating action reflected the 'dramatic worsening of already difficult auto industry conditions', noting that the share price of all three had fallen due to renewed competitiveness by American producers.
'The unexepected appreciation of the yen by 17 per cent in the last few months will further erode the earnings of the Japanese auto companies as well as erode their competitiveness if they continue to raise prices to offset the falling yen,' the agency warned.
According to recent research by DRI/McGraw-Hill, the consultancy group, every one yen increase in the strength of the Japanese currency costs Toyota dollars 50m ( pounds 33m).
Honda, which relies more heavily on exports than Toyota or Nissan, has already taken action to try to counter problems caused by the strong yen by raising prices in the US and cutting margins earned by dealers in the UK.
Japanese car workers, who have become accustomed to a jobs-for-life culture, have not yet had to face the prospect of redundancy.
However, many have lost the traditional 20 per cent overtime part of wage packets as companies have tried to cut costs.
Standard added that an additional problem facing the Japanese motor companies 'is that weak domestic demand is proving to be persistent. The European market, too, remains weak.'
Its credit watch covers the triple-A long-term rating of Toyota and its financial subsidiaries in the US, the Netherlands, Australia and Canada.
Also affected are the single-A long-term and A-1 short-term ratings of Nissan and the A-1 short-term rating of Honda for both parent companies as well as two subsidiaries each in the US and one each in the Netherlands.
Standard will meet the three companies before Christmas to review trends in earnings and their plans to cope with the stronger yen, to reduce costs and strengthen balance sheets.
The Japanese July automobile export figures mark the fourth consecutive month of year-on-year declines.
Car exports bore the brunt of the fall in July, dropping by 14.3 per cent to 311,839. Shipments of trucks fell by 13.4 per cent to 86,305, although buses edged up by 0.7 per cent to 6,034.
Exports to the European Community decreased for the sixth straight month, and markets in the Middle East, North and South America and Africa also declined.
The dual problems with currency and falling domestic demand are also hitting the steel industry.
Nippon Steel, the biggest producer in Japan, yesterday warned it was likely to incur a pre-tax loss in the year to March - its first in seven years.
Steel industry observers in Japan reckon that Nippon could lose as much as Y10bn in the six months to September, more than double previous estimates.
That would compare with the Y14.1bn profit the company made in the same period last year.
In 1987, the company lost Y12.6bn, principally due to the Japanese currency's surge against the dollar.