The Federal Trade Commission is investigating the Campbell Soup Co. in an effort to determine whether it has violated antitrust laws to maintain an overwhelming share of the canned soup market.

The commission has asked a federal judge in Pittsburgh to lift a protective order, which has prevented the disclosure of documents accumulated in connection with a civil antitrust case against Campbell brought four years ago by H. J. Heinz.

Although the Heinz suit was settled last May in an out of court settlement, the documents have been put under protective order and only an FTC subpoena for many of them prevented the destruction of thousands of pages of materials.

Campbell, for many years has dominated the canned soup industry, accounting for 30 to 90 percent of the marekt, industry sources said yesterday.

During the 1979 fiscal year, Campbell had worldwide sales of more than $2.2 billion.

The FTC's probe has been held up by a motion by Campbell to quash the subpoena, and lawyers for both sides may receive a court ruling on the motion today in federal court.

A Campbell's spokesman said yesterday that the company was advised in late December that it was the target of a nonpublic FTC investigation, but would not comment further on the commission action.