The brief by the U.S. solicitor general contradicts a brief filed by the Securities and Exchange Commission, which argued for shareholders' rights to sue those third parties. "Allowing liability for a primary violation under the circumstances presented here would constitute a sweeping expansion of the judicially inferred private right of action" under securities law, wrote Solicitor General Paul Clement.

Such a move could expose customers, vendors and others to "billions of dollars in liability when issuers of securities make misstatements to the market," he wrote.

Clement wrote that allowing third parties to be sued would "vastly expand liability in unpredictable ways."

The case, Stoneridge Investment Partners v. Scientific-Atlanta, Inc., has attracted considerable interest from lawmakers and industry associations. On Tuesday, Senate Banking Committee Chairman Christopher Dodd, D-Conn., asked President Bush to back the SEC's position.

Meanwhile, Securities Industry and Financial Markets Association President Marc Lackritz said allowing third parties to be held liable would result in skyrocketing litigation costs for companies.

"Investors already receive substantial protections under the law, and the Securities and Exchange Commission and other securities regulators are already armed with all the necessary regulatory tools to recoup lost money for investors," Lackritz said.

Similarly, the U.S. Chamber of Commerce, a business trade group, had urged the court to reject the expanded liability, known in legal terms as "scheme liability."

"Congress authorized the SEC to enforce securities laws against third parties and disburse funds to harmed investors," said Robin Conrad, executive vice president of the National Chamber Litigation Center.

"The Supreme Court should not upset that legislative decision by allowing class action lawyers to increase litigation risk and further hamper the competitiveness of American markets," Conrad said.

The Supreme Court is scheduled to hear the case in its fall term.