The revised agreement between Google, authors and publishers on digitising books still does not address copyright and monopoly concerns, says the US Department of Justice (DoJ).
The original October 2008 agreement was revised in November 2009 after continued objections by the DoJ, competitors in the digital books market, some US authors groups and several European countries.
The revised agreement is to be reviewed by a New York federal court on 18 February, but the DoJ has told the court it is still not happy.
The DoJ said despite progress in protecting unknown copyright holders and reducing the number of non-US books covered by the agreement, Google would still have a monopoly on digital books.
The DoJ said it remains committed to working with the parties to find a way for copyright holders to allow the digital use of their works by Google and others.
US consumer watchdogs have welcomed the DoJ's objections to the revised settlement.
In January, the National Writers Union, the American Society of Journalists and Authors and the Science Fiction and Fantasy Writers of America also rejected the amended agreement.
The groups criticised the amended agreement on several points, including the complexity of opting out and the proposed regulatory board that would override individual contracts.
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Google has confirmed plans to launch a digital bookstore before the end of the year that will step up competition with Amazon and Apple.