On the eve of the Senate Foreign Relations Committee debate on economic sanctions against South Africa, the office of the U.S. special trade representative has negotiated a tentative bilateral agreement with the South African government increasing the rate of textile exports to the United States.

Democratic proponents of sanctions quickly denounced the action.

"This is lunacy," said Rep. William H. Gray III (D-Pa.). "I don't think it's proper."

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said, "It is outrageous that, at the same time the American people are demanding an end to our economic support of apartheid, the administration has secretly agreed to increase that support."

Sen. Edward M. Kennedy (D-Mass.), in remarks prepared for delivery on the Senate floor today, said, "The administration does South Africa a favor, sticks its finger in the eye of millions of U.S. textile workers, and guarantees an increasing share of the U.S. market for South Africa."

The agreement between the United States and South Africa approving a 4 percent rate of increase in textile exports to the United States followed standard procedure. It was initialed by Liz Cummings, the assistant chief textile negotiator, on June 27. The arrangement is scheduled to go into effect on Sept. 1 after a "memorandum of agreement" is exchanged.

Ron Sorini, the deputy chief textile negotiator, said, "This was done solely in textile terms, not in broader political terms."

The 4 percent increase granted South Africa is greater than the rate increases granted this year to Taiwan, South Korea and Hong Kong -- the major textile exporters to the United States.

More than 50 countries are signatories to the Multi-Fiber Agreement, which establishes rules for the textile trade. Within this framework, the United States negotiates bilateral agreements with specific trading partners. South Africa, however, is not a member of the Multi-Fiber Agreement.

According to Kennedy's statement, "South African textile exports to the United States doubled between 1984 and 1985, but no provision of U.S. law of our multilateral trade agreements required that we accept this surge or, in fact, negotiate at all."

Congressional interest in the new U.S.-South African deal first surfaced last Friday.

Gray telephoned Special U.S. Trade Representative Clayton Yeutter. "I finally reached him late Friday afternoon," said Gray. "He did not recall any specific agreements at that time."

Yesterday, Yeutter informed both Gray and a Kennedy aide of the existence of the agreement.

Sorini said: "The agreement is not public . . . . I'm not at liberty to give details."

"I don't like it at all," said Gray. "We ought to be cutting it trade with South Africa all off. I don't how they could be talking about this."