Google's ambition to create the largest body of human knowledge on the internet by scanning millions of library books and turning them into a massive digital publishing venture is prompting growing opposition from authors and legal experts who object to its scope and copyright implications.

Opponents and supporters of Google's plans are lining up for a showdown that will come to a head on 4 September, the deadline for submissions to be lodged with a Manhattan court that is reviewing the scheme, known as Google Book Search.

The court is considering whether or not to give the go-ahead to the settlement of a class-action suit that Google reached with key publisher and writers' groups last October. The settlement, approved by the Authors Guild and the Association of American Publishers, provides for a pot of $125m (£75m) that Google has agreed to pay to cover copyright infringements it had already committed by scanning books online.

The settlement would also give writers and publishers the equivalent of 63% of future revenues generated by sales of digital books and other income, while Google would keep the remaining 37%.

But several groups and individuals are continuing to protest about the deal, saying that it rides roughshod over authors' rights and awards Google a monopoly over a huge body of work.

The latest objection, filed with the Manhattan court today, comes from a Washington-based lawyer and writer who specialises in class-action law and monopolies. In his 47-page complaint, Scott Gant argues that potentially millions of authors in America and around the world are being coerced into accepting the deal without being fully informed about its implications.

"Anyone taking part in this project should be doing so as a conscious choice to participate knowing fully what they are doing. In fact, people are being forced to hand over to Google some of their intellectual property often with no understanding of what that means," Gant said.

Under US class-action law, authors and publishers who do not specifically opt out of the settlement are deemed to have signed up to it. But Gant points out that as an author himself — he wrote a book on the digital information revolution called We're All Journalists Now: The Transformation of the Press and Reshaping of the Law in the Internet Age — he has never received any legal notice about the case.

Google announced its plans five years ago, arguing that the project to build up an online archive of millions of books that are out of print was part of its mission to "organise the world's information". It has already scanned at least 7m books, using cameras able to convert up to 1,000 pages an hour.

Most of the books, which must have been published before 5 January this year, have come from libraries and publishers in the US. Google has so far struck partnerships with 29 of the world's biggest libraries, including those of Harvard and Stanford and the Bodleian in Oxford.

Other opponents of the deal include the National Writers Union and the American Society of Journalists and Authors. Eighteen professors within the University of California system have also written to the court objecting that the settlement fails to protect the interests of academic authors and puts profit before the public's right to information.

But the Association of American Publishers, which originally sued Google in 2005 for breach of its members' copyright but has since signed up to the settlement, argues the deal is good for publishers, authors and the public.

"This will allow the public to become familiar with millions of books that have been out of print and unknown to them or unavailable," said the association's Allan Adler.

He said that groups still holding out objections were making a fundamental misreading of the deal. Writers and publishers will still have the right to decide whether or not to participate.

The mass of conflicting opinions will be drawn together in the New York court ahead of a hearing on the settlement in October. The US justice department is also looking into the Google scheme to see whether it is in breach of competition laws.

A Google spokeswoman said last night: "We're excited about the groundbreaking agreement we reached with authors and publishers last year. If approved by the court, this settlement stands to unlock access to millions of books in the US while giving authors and publishers new ways to distribute their work.  

"Google was founded on the principle of making information more accessible to more people. From the beginning, we've envisioned a future where students, researchers, and book lovers could all discover and access the world's books online.  We believe that this agreement represents a giant step toward realizing that vision."