The first crack in the Bell System's united public posture on the company's forthcoming breakup emerged today with a warning from the chief executive of Pacific Telephone & Telegraph Co. that divestiture will leave his company in an unsound financial condition.

Pacific Chairman Donald E. Guinn, in an affidavit filed in federal court in Washington, said the restructuring of the company's balance sheet necessary for the January 1984 breakup of American Telephone & Telegraph Co. will leave it burdened with too much debt.

"I am of the opinion that the plan leaves Pacific in a disadvantageous financial condition and does not divest Pacific in a 'sound-balance-sheet condition,' " Guinn said.

The affidavit was among eight submitted to U.S. District Judge Harold Greene in conjunction with the filing yesterday of AT&T's plans for divestiture. Of the seven affidavits from chief executives of the regional telephone companies that will be spun off from AT&T a year from now, only Guinn's raised serious concerns about the massive AT&T plan.

Guinn's views were echoed in essence by California Public Utilities Commission Chairman John Bryson, who said he is "concerned" that the AT&T reorganization plan would leave Pacific "in such a weakened position that California telephone service would suffer."

"AT&T must accept its commitments to leave Pacific and the other Bell operating companies in sound financial condition," Bryson said.

Pacific is in the worst financial condition of the current 22 AT&T local telephone companies that will be combined into the seven new independent regional companies. Pacific and its wholly owned subsidiary, the Bell Telephone Co. of Nevada, will be one of the seven regionals.

Furthermore, the company's debt-to-equity ratio rose from 48.6 percent in 1973 to 59.1 percent in 1980. As of the close of 1981, the company was paying about 9.9 percent for its debt, above the Bell System average of 8.6 percent. AT&T said Pacific's debt ratio would be down to 50 percent at the time of divestiture, although Guinn said the correct figure would be 54.4 percent when preferred stock is taken into consideration. The comparable figure for the other divested phone companies will be 45 percent, AT&T said.

The California commission has been in a series of fierce rate fights with Pacific, part of the company's difficulty. It also owes $1.2 billion in back federal taxes as a result of a dispute between the Bell System and the California commission over accounting practices. Pacific says that figure will rise to $2 billion over the next four years. Legislation before the lame duck Congress would ease the tax burden.

Without more assistance from AT&T, it would "be virtually impossible for Pacific to meet its normal capital needs" and "needed telephone service would have to be curtailed," Guinn said.

AT&T Chairman Charles Brown said in a separate affidavit that, in acting to lower Pacific's debt ratio, the company has done all it can. Calling Pacific's problems "serious," Brown said the "cure can only come from fair regulation of Pacific in the future."