The US federal court overseeing Google's settlement over its book-scanning programme is giving authors four more months to opt out of the deal.

Instead of a May 5 deadline, the court has ruled that authors now have until September 4 to review the ground-breaking agreement.

Under the Google Print Library Project, snippets from millions of out-of-print but copyright-protected books have been indexed online. Google has called the project, which also scans public-domain works, an invaluable chance for books to receive increased exposure.

But in a class-action suit filed in 2005, the Authors Guild alleged that Google was "engaging in massive copyright infringement". Within weeks, publishers also sued.

In October 2008, Google and the publishing industry agreed to settle their battle. The settlement, a template for better relations between the search giant and publishers, calls for Google to pay $125 million while developing online sales opportunities for scanned books that turn up in Google searches. Google would get 37 per cent of future revenue and publishers and authors would share the rest.

Google would also pay for the millions of copyrighted books already scanned — $60 per complete work to the rights holder — and for the legal fees of the Authors Guild and publishing association. The settlement would become a template for digital rights for years to come.

In November 2008 a judge set a June 2009 date for a final settlement and hearing to decide whether the deal is fair, reasonable and adequate. The extension pushes back the final hearing on the settlement's approval to October 7.

The delay follows a request from the heirs and trusts of a number of authors, including John Steinbeck and Philip K. Dick. In a statement, a Google spokesman said: “As we’ve said previously, the settlement is highly detailed, and we want to make sure rights holders everywhere have enough time to think about it and make sure it’s right for them.”

But there may be another hurdle to the deal's approval. The Justice Department is reviewing whether Google's settlement violates antitrust laws, The Wall Street Journal reported.

The US Justice Department recently contacted Google and other parties for details on the settlement, the newspaper said, noting that it is unclear whether the agency would try to block the deal.

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