A final judgment or decree heretofore or hereafter rendered in any civil or criminal proceeding brought by or on behalf of the United States under the antitrust laws to the effect that a defendant has violated said laws shall be prima facie evidence against such defendant in any action or proceeding brought by any other party against such defendant under said laws as to all matters respecting which said judgment or decree would be an estoppel as between the parties thereto: Provided, That this section shall not apply to consent judgments or decrees entered before any testimony has been taken. Nothing contained in this section shall be construed to impose any limitation on the application of collateral estoppel, except that, in any action or proceeding brought under the antitrust laws, collateral estoppel effect shall not be given to any finding made by the Federal Trade Commission under the antitrust laws or under section 45 of this title which could give rise to a claim for relief under the antitrust laws.