"This Court should reject the Proposed Settlement in its current form and encourage the parties to continue negotiations," reads a filing the DOJ submitted to the U.S. District Court for the Southern District of New York late on Friday.

The proposed settlement must be modified by Google and the plaintiffs so that it complies with U.S. copyright and antitrust laws, the DOJ said in its filing, a 32-page Statement of Interest.

The DOJ's opinion is likely to be seen a major blow against Google, which has been in court over this matter for about four years and is eager to get the case settled.

The DOJ had until Friday to submit a written report to the court on the findings of a formal investigation over whether the proposed agreement violated U.S. laws.

Rumors that the DOJ had started reviewing the proposed agreement surfaced in April. The DOJ confirmed that it was conducting a formal investigation in July.