The Swiss government said yesterday it was prepared to turn over to U.S. prosecutors documents the Swiss seized more than a year ago from Marc Rich & Co., the big commodities trading firm that pleaded guilty in New York to tax evasion charges last month.

The firm and Clarendon Ltd., its New York subsidiary, paid $200 million in back taxes, fines and foregone tax deductions -- the largest amount ever recovered in a tax evasion case.

Morris Weinberg Jr., the assistant U.S. attorney in New York who handled prosecution of the Marc Rich case, said the documents might be useful if Marc Rich himself and his associate Pincus Green are ever brought to trial. Rich, who renounced his U.S. citizenship last year, is living in Switzerland, where the firm bearing his name has its headquarters, but the Swiss have refused an extradition request from the U.S. government.

The Swiss seized the firm's documents in September 1983, after the firm acceded to a subpoena issued in New York City to supply the documents. The Swiss said that it would violate the law for the trading company to turn over secret business documents and said the U.S. government could apply for the documents under treaty procedures.