NEW YORK, JAN. 7 -- The number of corporate credit ratings lowered by Standard & Poor's Corp. in 1990 nearly doubled to an all-time high, shattering the record set in 1989, S&P's chief rating officer said today.

Branding the fast-track 1980s "a decade of lost opportunities," Leo C. O'Neill, president of Standard & Poor's Ratings Group, warned of more painful times to come unless bond issuers put their financial houses in order.

"This year will be a critical test to determine whether the decade-long deterioration of credit quality in the 1980s continues," O'Neill said in a speech at S&P headquarters in Manhattan.

In the latest indication of the nation's deepening economic problems, S&P reported that 1990's 768 downgrades in corporate long-term ratings was quadruple the number of upgrades and nearly double 1989's record number of 419 downgrades.

Ratings by S&P and Moody's Investor Services Inc., the other major ratings service, are used to help bond buyers weigh the riskiness of bond issues and have enormous influence on the amount of interest a bond issuer must pay. The ratings are considered an important gauge of a company's fiscal soundness.

O'Neill warned that the uncertainty surrounding the Persian Gulf conflict clouds the credit picture for 1991. "An outbreak of armed conflict would likely lead to higher oil prices, heighten fears of inflation and deepen the recession," he said. "As a result, creditworthiness could deteriorate beyond S&P's current expectations."

The potential for a more painful recession, O'Neill said, could lead to a further credit deterioration, particularly among the nation's lending institutions and insurance firms.

O'Neill said a slightly better picture faces the municipal bond markets, but he said widespread anti-tax sentiment will force public officials to "address revenue shortfalls on a timely basis."