Waksal, appearing in federal court in New York, pleaded to conspiracy and wire fraud for evading $1.2 million in sales taxes, joining former Tyco International Ltd. CEO Dennis Kozlowski, who was indicted on similar charges.

Each of the two new charges, which are not related to the biotechnology company Waksal founded, carries a possible prison term of five years.

The scheme involved nine works, by artists such as Roy Lichtenstein and Willem DeKooning, valued at about $15 million, according to prosecutors. Waksal allegedly had the works shipped to an ImClone manufacturing facility in New Jersey to avoid New York's sales tax.

The paintings were then redirected to his New York apartment, said U.S. Attorney James Comey. At other times, Waksal did not even have the paintings shipped to New Jersey, he simply had the bill sent there, he said.

"This wasn't about art, but about greed," said Comey.

One of the paintings alone was worth the amount of the tax Waksal evaded.

"That means taxpayers paid for it," Comey said.

INSIDER TRADING SCANDAL

Waksal was at the center of an insider-trading scandal that involved family and friends, including home decorating expert Martha Stewart.

He attempted to sell a huge block of his company stock in late December 2001 just before U.S. regulators rejected ImClone's application to sell its experimental cancer drug Erbitux, news that sent ImClone shares plummeting.

He is also accused of tipping off his daughter, Aliza, and father, Jack, that Erbitux would be rejected before the company released the information, giving them time to sell their shares before the stock fell. Prosecutors are seeking to take a total of $10 million from Aliza and from Waksal's parents.

Waksal's sentencing date was postponed to May 29 from March 17 because of the new charges. Waksal has arranged with the government to pay the taxes owed, according to his lawyer, Mark Pomerantz.

Waksal has provided information to prosecutors about other people in the insider-trading investigation, but those did not include Martha Stewart, Pomerantz said.

Waksal, 55, whose Manhattan loft was filled with expensive artwork, pleaded guilty in October to six of the 13 counts for which he was indicted in the scandal.

"I look forward to the day when I can put all of this beyond me and move on with my life," he said in a prepared statement.

Tax-evasion charges on luxury items have brought down other rich and powerful business leaders.

Tyco's Kozlowski was indicted by state prosecutors for allegedly dodging $1 million in New York sales taxes on the purchase of works by such masters as Monet and Renoir. He is accused of having empty boxes shipped to his New Hampshire offices in a ruse intended to show that the artwork was leaving New York, thus avoiding New York's higher sales tax.

Real estate mogul Leona Helmsley in the early 1990s was accused by New York officials of evading sales tax on expensive jewelry, setting the stage for more serious probes of federal tax evasion that led to her conviction and imprisonment.

ALREADY FACING PRISON

Waksal already faces up to 65 years in prison for the six counts to which he already pleaded guilty: securities fraud, perjury, conspiracy to obstruct justice, obstruction of justice and bank fraud.

He did not plead guilty to charges of conspiring with his daughter to trade on inside information and has consistently tried to shield his family from additional scrutiny.

Shares of ImClone rose $1.02, or 7.7 percent, to $14.35 on Nasdaq after earlier running up more than 10 percent.

The ImClone stock has risen over the past few weeks on expectations of positive clinical trial data for Erbitux.

German drugmaker Merck KGaA (MRCG.DE), ImClone's European partner on Erbitux, said on Monday it would spend about $324 million on a plant to manufacture the drug.

"The fact that they're making a commitment of that sort means that they certainly think it's positive data," said Jim McCamant, an analyst for Moors Cabot. (Additional reporting by Gail Appleson and Toni Clarke)