Publishers and agents representing the authors J. K. Rowling and Ken Follett were battling last night to get free copies of their novels removed from a Californian website that claims to be the most popular literary site in the world.

Scribd.com attracts 55 million visitors a month, many drawn by the chance to download versions of books by popular authors that have been uploaded on to the website without the consent of the writer or publisher.

A search of Scribd by The Times yesterday found copies of Harry Potter and the Half-Blood Prince and Ken Follett’s most recent novel World without End among many bestselling titles, raising fears that the piracy affecting the music industry may have spread to books.

When presented with a list of links to various Harry Potter books, Neil Blair, J. K. Rowling’s lawyer at the Christopher Little literary agency, said that Scribd did not have permission “and what you have identified are infringing listings which we were aware of and actioning”.

Ken Follett’s publisher, Macmillan, was unware that World without End had been uploaded on to the Scribd website for more than five months, and had been read more than 500 times there. Macmillan said it was “now looking into this”.

Also on the site were copies of titles by Nick Hornby and John Grisham, uploaded seemingly without permission. These books can be downloaded to a home computer or an electronic books reader, and then printed out.

Peter Cox, a literary agent and editor of the Litopia blog, said: “These people are pirates. We don’t have to give in to this. We can’t afford to make the same mistakes the music industry did.”

Scribd was set up by Trip Adler and Jared Friedman, Harvard students in their early twenties, and in two years has become the “YouTube for books”, helped by $12 million (£8.4 million) of financing. It makes money from advertising but pays no royalties to authors. It has rapidly become the most popular site for reading books online and 50,000 books and documents are uploaded on to Scribd every day. The site was used by the Obama campaign to publish policy documents, with the aim of giving people access to information directly, bypassing the filter of the news media.

Mindful of copyright concerns, Tammy Nam, a spokeswoman for the San Francisco Scribd, says that it operates a “notice and takedown system”, where it removes books if their publishers demand it. She said: “If we get a request we usually respond in 24 hours.” This makes the site compliant with the US Digital Millennium Copyright Act, which means that the site is not held liable for actions of its users of which it is not aware.

Critics say that this is not enough, because authors and publishers are not always aware that people are uploading books illegally.

Scribd has many legitimate uses, and the company is testing book giveaways with some American publishers, including Random House. Tess Gerritsen’s book The Surgeon was posted on the site with permission, and was read 30,000 times in three weeks. “Her readers have typically been women in their forties and fifties; this was a chance to get the book in front of a broader audience,” Ms Nam said.

However, other publishers are unimpressed. John Makinson, the chief executive of Penguin books, said that his company was not participating. “We do have a concern about the amount of free content on the web, and the impact that will have on the consumer’s perception of the value of books,” he said.

Literary theft on the internet

— In July 2007 pages from Harry Potter and the Deathly Hallows appeared on the internet four days before the book went on sale. A Chinese translation appeared online weeks before the official Chinese-language version reached bookshops

— In 2003 an e-mail claiming to contain an early copy of the chef Jamie Oliver’s next book flashed around the world. It was a hoax but it did include a number of Oliver’s recipes from previous collections

— Last summer the American author Stephanie Meyer said she would not continue with her latest novel in the Twilight series after an unfinished draft of the book, Midnight Sun, was leaked on to the internet. She said the project was on hold indefinitely