Tax-shelter promoter Charles Agee Atkins was convicted in Manhattan yesterday of conspiracy to commit tax fraud in a scheme that generated more than $1.3 billion in phony trading losses for cash-rich celebrities and other influential investors.

Atkins' scheme, which prosecutor Stuart E. Abrams said was the largest of its kind in history, enabled investors to claim a total of at least $350 million in false deductions on income tax returns. It also enabled Atkins to take as much as $300,000 a week out of his tax-shelter operations while still in his 20s, Abrams charged.

The 400 investors included the Tisch brothers -- Laurence, chief executive of CBS Inc., and Preston R., the postmaster general -- as well as Michel David Weill, chief executive of Lazard Freres & Co., a major investment banking firm; television producer Norman Lear; actors Michael Landon and Sidney Poitier, and the late artist Andy Warhol. None was charged with criminal wrongdoing, but U.S. Attorney Rudolph W. Guliani and IRS officials have said that they were liable for back taxes, interest and possible penalties.

Abrams said the guilty verdict will "substantially enhance" the IRS's ability to collect civil penalties.

Atkins, 33, who was convicted of 27 additional tax-related charges, faces a maximum prison term of 86 years, a fine of up to $145,000, and a bill for the costs of prosecution. He is a former research fellow at the Brookings Institution in Washington.

A U.S. District Court jury also convicted two codefendants: William S. Hack, 62, a New York tax-shelter promoter and graduate of Harvard Law School, and Ernest M. (Mike) Grunebaum, 52, of Chappaqua, N.Y., who became head of trading for Atkins and who used a shell corporation, NYH Equities, for $3 billion in bogus deals with Atkins' operation.

Atkins and Hack were found guilty on all of the counts on which they were indicted March 25. Hack faces a maximum penalty of 17 years and $30,000. Grunebaum, convicted on 14 of 16 counts, faces a maximum of 92 years and $160,000. Sentencing has been set for Feb. 10.

The three were found guilty of conspiring to cheat the IRS by entering into transactions in government securities that were prearranged solely to create tax deductions.

Atkins' counsel, former Assistant U.S. Attorney James A. Moss, said he would appeal. "The propriety of the tax deductions alleged to be false ... was a key legal issue ... upon which Mr. Atkins intends to base his appeal. It is my expectation that ... he will be vindicated," Moss said.

"Hundreds of millions of dollars in taxes were evaded by a bunch of people who just sat around writing up pieces of paper to take phony tax deductions," Abrams told the jury in his closing argument last week.

Atkins, his codefendants and the unindicted coconspirators "were engaged in a very simple tax fraud," he continued.

Atkins was 24 when he and Washingtonian Edward A. Markowitz joined up in 1978 to start what became The Securities Groups, which Atkins characterized as an investment banking firm. In reality, the prosecution charged, it was Atkins' "piggy bank," boosting his net worth from $100,000 to more than $66 million by the end of 1982.

Atkins bounced Markowitz in 1979 in a showdown in the empty New York offices of Ashland Oil Co. Atkins' father, Orin E., who was chairman of Ashland at the time.

Markowitz, who cooperated with and testified for the government in the Atkins case, pleaded guilty in April 1985 to tax-fraud charges arising from the sale of more than $445 million in phony tax writeoffs in similar operations he set up after splitting with Atkins. At the time, his was the largest criminal tax-fraud case ever prosecuted. He is awaiting sentencing and faces a maximum penalty of 16 years in prison and a $310,000 fine.

TSG was the umbrella for three tax shelters: The Securities Group, The Securities Group 1980 and The Monetary Group. Most investors were limited partners who were told that at the end of each year in which they put up $1 in cash, they would have $4 in tax deductions.

TSG was "a full-service tax fraud operation" that by 1982 had about $24 billion worth of securities, but only about $100 million in cash, Abrams said. TSG went bankrupt in early 1983 after its accounting firm, Arthur Young & Co., resigned the account. Atkins, now of Amelia Island, Fla., filed for personal bankruptcy in 1984 and testified during his trial that he is destitute.

The trial began Oct. 8 before U.S. District Judge Edward Weinfeld who, after being injured in a fall just before closing arguments were to begin, turned the final phase of the case over to Judge Morris E. Lasker. It is uncertain which judge will impose sentences.

In a jolt for the defendants a few days after the indictment was handed up, four general partners in TSG pleaded guilty to various tax-fraud charges. They are Steven R. Hageman, Kenneth T. Kaltman, Robert V. Gubitosi and Joseph J. Riley. The government called all four to the witness stand, and their testimony proved to be devastating to the defense.

Riley, who had been responsible for trading activities in Treasury bills in both cash and futures markets and who now works in a coin-operated laundry, described Atkins as "the guru ... who approved every tax transaction."

A key Atkins defense was that he was unaware of improper activities in TSG.

When the prosecution asked Gubitosi to state the purpose of the deals with NYH Equities, the Grunebaum shell corporation, he said it was "to generate tax losses for {TSGs}." Gubitosi also testified: "Grunebaum indicated to me that he would put positions on with NYH Equities that would yield $50 million worth of tax losses for the firm {TSGs} at a cost of approximately $50,000."

William J. McCormick, an Arthur Young partner, testified that he became suspicious of NYH Equities because it "didn't look real," since he couldn't find it at its listed address and its phone number "turned out to be the National Bank of Canada."