Standard & Poor's Corp. broke with its major competitor among credit rating services yesterday by retaining the gilt-edge triple-A ranking for a large number of American Telephone & Telegraph Co. subsidiaries, many of which had their ratings downgraded two weeks ago by Moody's Investors Services.

But it downgraded the credit rating of 11 other Bell System companies, including three of the four Chesapeake & Potomac Telephone Companies. The fourth, C&P of Virginia, kept its AAA ranking.

Standard & Poor's said it has not completed its analysis on AT&T itself and on its manufacturing arm, Western Electric, as part of a general re-evaluation of creditworthiness of the giant system, brought about by the approaching court-sanctioned breakup of Ma Bell.

The split between the two premier rating companies leaves some confusion within the investment community, which relies on them for guidance on companies' ability to pay back major loans taken out in the form of bond issues.

The Moody's downgrading tarnished AT&T's long-standing high credit rating and at first lowered the resale value of hundreds of millions of dollars' worth of telephone company bonds, which had been considered the ultimate in blue-chip investments. Since that announcement March 10, however, telephone bond prices recovered as most investors decided that "Moody's was wrong," one Wall Street trader told Dow Jones News Service. Another bond analyst said Moody's took a worst-case scenario in dropping Bell System ratings.

Moody's said it lowered its rating of AT&T and 23 of its 24 local operating companies because the coming breakup would weaken them all financially. Its ranking left no company in the Bell System with a triple-A rating.

Standard & Poor's, though, said it kept the high rating for the companies it studies in recognition of the Bell System's dominant business position.

"Neither divestiture nor the heightened regulatory and competitive risks will significantly impair the ability of Bell operating companies to extend the record high customer service and superior creditor protection over the longer term," Standard & Poor's said.

In all, Standard & Poor's reaffirmed the top-grade AAA rating of eight companies and kept a ninth in the second-ranked AA-plus rating. It also raised its appraisal of the creditworthiness of the lowest-ranked Bell System operating company, Pacific Telephone & Telegraph Co., from A-minus to A-plus. Moody's had lowered that company's rating to Baa-1. The two companies have different ways of expressing rankings.

Locally, C&P Telephone Co. of Washington and C&P Telephone Co. of Maryland were downgraded from AAA to AA, while C&P of West Virginia had its credit rating dropped even further, from AAA to AA-minus. Moody's had downgraded all four C&P companies from the Aaa ranking to Aa-3 for C&P of Virginia, A-2 for C&P of Washington and C&P of Maryland and A-3 for C&P of West Virginia.

AT&T praised the Standard & Poor's ratings as "realistic."

"S&P correctly perceives that the companies are now subject to a greater risk. It is significant, however, that the agency still rates all these bonds as being of strong investment-grade quality and saw fit to maintain the ratings of nine of the companies and even upgrade one," said AT&T Vice Chairman and Chief Financial Officer William S. Cashel.

He had called Moody's downgrading "harsh" and said they were "not borne out by the facts."