WASHINGTON — The Supreme Court on Monday rejected an appeal by Microsoft Corp. and a unit of Best Buy Co. Inc. in a lawsuit alleging the two companies fraudulently signed up customers for Microsoft's online service.

The companies asked the justices to overturn a May ruling by the 9th U.S. Circuit Court of Appeals, based in San Francisco, that allowed the lawsuit to proceed. The Supreme Court's action lets stand the appeals court's ruling.

The class action lawsuit argues that a joint venture between the two companies violated the Racketeer Influenced and Corrupt Organizations Act, or RICO. RICO provides for triple damage awards in civil cases.

The U.S. Chamber of Commerce said in court papers that the use of RICO in civil cases has gotten out of control and urged the court to take the case in order rein in its use.

The case is Microsoft Corp. v. Odom, 07-138.

Shares of Microsoft fell 7 cents to $30.10 in early trading Monday.