A leading debt rating service said yesterday it may downgrade $54 billion in outstanding debt issued by General Motors Corp. because of growing competition in the global auto industry, lagging U.S. car sales and the possibility of a recession.

Moody's Investors Service of New York said it began a two-month review of GM's debt worthiness. It will look at debt issued by GM, General Motors Acceptance Corp. and other subsidiaries.

GM's long-term debt now has a Moody's rating of Aa1, meaning "high quality." That is one step below Aaa, Moody's highest, or "prime," rating. It appeared that the GM's rating might be changed from Aa1 to Aa2 or even Aa3, but would remain in the "high quality" category.

But a downgrade of any extent would have an impact on GM, the nation's largest industrial corporation. Debt ratings generally affect the interest rates that corporations have to pay to raise money; the better the rating, the lower the cost to the company.

Harold H. Goldberg, chairman of Moody's corporate rating committee, said that after five years of expansion, the nation's economy was "showing signs of fatigue and consequently, it's only a question of time before the gross national product's growth continues to decline and we're going to see a recession. It's inevitable."

Goldberg said the stock market collapse in October was not responsible for Moody' decision to review the GM ratings, which had been in effect for three years. However, he acknowledged that the stock market losses could result in "less enthusiasm by the buying public."

In a post-Oct. 19 study of the auto industry, Merrill Lynch said it looked for weaker 1988 and 1989 sales than previously anticipated. The researchers said they expected passenger car production in North America to decline from 7.9 million cars in 1987 to 7.65 million in 1988 and 7.55 million in 1989.

Moody's said it would focus on "GM's ability to regain momentum and to strengthen its long-term business position in the face of an increasingly difficult operating environment."

Moody's noted GM was trying to cut costs and narrow product focus to meet increasing global competition.

The rating firm said it would review GM's belt-tightening efforts and the impact they might have on GM's profit margins.

GM responded to Moody's action by saying that "General Motors is one of the most financially sound industrial organizations in the world."

"Our investments in new products, modernized facilities, a trained work force and advanced technology provide the necessary foundation for sound, long-term financial performance. In addition, we have instituted a number of measures to significantly reduce our costs and simultaneously improve our competitiveness worldwide," GM said.

The Moody's review covers GM's senior long-term debt, rated at Aa1; s GMAC's senior debt and grantor trusts, rated at Aa1, and its subordinated debt, rated at Aa2; and the guaranteed Eurobonds of GMAC of Canada, GMAC of Australia (Finance) Ltd.; and GMAC (UK) Finance PLC, all rated Aa1.

GMAC's Prime-1 rating for its commercial paper is not under review.