The organisation, representing American publishers of academic journals and scholarly books, claims the project has financially troubling consequences.

It believes it could undermine sales of works publishers own the rights to and it has written to Google to say so.

Last year Google announced deals with four of the world's top universities.

Google's $200m (£110m) plans were announced in December. It aims to put 15 million volumes online from four top US libraries - the libraries of Stanford, Michigan and Harvard universities, and of the New York Public Library - by 2015.

It will also scan in out-of-copyright books from the UK's Oxford University.

Erudition online

The idea is to make millions of important but previously inaccessible texts available to researchers everywhere, with a few clicks of a computer mouse.

The plan has its supporters. The head of Oxford University's library service said the project could turn out to almost as important as the invention of the printing press.

Google meanwhile claimed its motives were purely altruistic, the realisation of a longstanding dream for the group's billionaire founders Sergey Brin and Larry Page who'd worked on a digital library project during their student days.

But from the start Google's recent plan met opposition. The letter to Google from the Association of American University Presses, which represents 125 non-profit-making academic publishers, is just the latest in a series of criticisms.

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The Association wants clarification on 16 questions and claims the book-scanning scheme "appears to involve systematic infringement of copyright on a massive scale."

Its members depend on book sales and other licensing agreements for the majority of their revenues. They are worried that if users can get the information they want from its books by searching them online, they won't bother to buy them.

Alternative to English

Other opposition has come from France, where there are fears that the Google project will enhance the dominance of the English language and of Anglo-Saxon ways of thinking.

France and several other European countries recently got European Union backing for a rival book-scanning project for works not in English.

Supporters of the Google project say copyright is protected because many of the works being initially scanned in are old texts not by living authors.

Google said in a statement on Monday that it offers protection to copyright holders. For newer books still in copyright, users will only see a list of contents and a few sentences of text.

Only older, out-of-copyright books from Oxford University and from the New York Public Library will be scanned into the Google system.