Standard & Poor's, one of the two major bondrating agencies, said today that it is uncertain whether American Telephone & Telegraph Co. and its 21 local phone company subsidiaries can maintain top-grade credit quality if AT&T splits off the subsidiaries as part of a proposed antitrust settlement with the Justice Department.

Standard & Poor's placed the parent company and the subsidiaries on a "credit-watch."

Most analysts think the antitrust settlement, which U.S. District Court Judge Harold Greene refused to approve today in Washington pending a review of its public-interest aspects, would be beneficial for AT&T -- which will retain its long-distance division, its Bell Labs and Western Electric manufacturing subsidiary -- because it permits AT&T to enter unregulated businesses. However, analysts said that the local phone companies have a murkier future because they will be unable to engage in anything but regulated activities in the provision of local phone service.

Some local phone companies have better credit ratings than others, although in the past all but Pacific Telephone & Telegraph have been treated as if they were AAA-rated (the top) because they were under the umbrella of AT&T, one of the nation's most solid companies.

Standard & Poor's said today it agrees that the split-up will be beneficial to AT&T, but said that AT&T's pledge to have the local subsidiaries on a sound financial footing at the time of divestiture might force the parent company to absorb some of their debts.

The rating agency did not alter the top ranking of AT&T and most of its subsidiaries, but said it would evaluate the situation during the next six months, during which AT&T is supposed to create a firm divestiture plan.

Investors have been reluctant to buy some lower-quality telephone company bonds, such as those of Pacific or Michigan Bell, since the antitrust settlement last Friday. For the most part, trading has been slow in both AT&T and Bell issues.

The key to the fate of the local companies depends on how the split-off takes place, analysts said. If they are split off into 22 separate local companies (part of Pacific Telephone will be turned into a separate company), the top-grade companies are likely to remain that way, but investors probably will be reluctant to provide funds to lower-quality companies.

Gerald Morgan, vice president and utility analyst at Bache Halsey Stuart Shields, said he think AT&T will divest the companies on a regional basis, creating perhaps six or eight new companies -- rather than one big company or 22 separate ones.