Microsoft and Yahoo are joining a coalition opposed to a legal settlement which gives Google the right to digitise millions of books.
According to Reuters, the companies will sign up to the Open Book Alliance, made up of libraries and non-profit organisations fighting against Google's plans. Amazon is also said to have joined.
Google’s opponents say that the deal, which could potentially create the world’s largest online library, would allow the company to monopolise the library system and set prices for institutions wishing to use its service.
They also say it would give Google the exclusive right to digitise so-called orphan works - books whose copyright ownership is unclear – potentially posing an antitrust concern.
A Google spokesman told Reuters that its critics were simply afraid of competition. "The agreement is not exclusive. If improved by the court it will expand access to millions of books in the US," he said.
The proposed deal was reached in October 2008 to settle a lawsuit filed by the Author's Guild in 2005. The guild alleges that Google committed copyright infringement when it began scanning books.
Google has agreed to pay $125 million to create a Book Rights Registry, which would allow authors and publishers to receive compensation from subscriptions or book sales, and an approval hearing has been scheduled for 7 October.
The deal is also being reviewed or investigated by the US Justice Department, several US state attorneys general, and the European Commission.