Newspapers are protective of their online content.
The Daily Mirror has joined an increasing number of newspapers to block news aggregator NewsNow from trawling its website.
The paper objects to NewsNow offering a subscription service that charges clients for digital press clippings, including links to Mirror stories.
The Sun and Times Online have also recently blocked the service.
NewsNow chairman Struan Bartlett called the move "an unacceptable restraint of freedom of expression".
He told BBC News that the aggregator - which collates links from various news sites - stopped linking to mirror.co.uk content in December 2009.
The Daily Mirror now uses robot.txt - a file which limits what search engines can index on a site - to block the aggregator.
The paper claims that by blocking the site it is protecting its investment in online material and has no objection to aggregators such as Google News, who do not make money from their feeds.
"Mirror Group invests significant sums to fund the journalism and produce the content that NewsNow has been selling by subscription for quite some time now," said Matt Kelly, Digital Content Director at Mirror Group.
"We'll thrive without NewsNow. The question is, will they thrive without us and our fellow content producers?"
Mr Bartlett said that NewsNow would be examining the impact of Mirror Group's decision on "internet freedom and the wider economy".
NewsNow has been unpopular in some newspaper circles since declining to pay for a licence from the Newspaper Licensing Agency (NLA) to distribute lists of news stories that it is paid to collate.
Services like this tend to be popular with companies and PR agents who wish to track mentions of their brands or products.
Andrew Hughes, Commercial Director of the NLA said it was "disappointing" but that NewsNow was within its rights to operate without the licence, which would have cost an estimated £36,000 ($58,000) including individual fees on a per client basis.
"We have no dispute with NewsNow," he told BBC News.
"But it's important to respect the creators of content and share the value that comes out of content creation."
The news comes as various newspapers - including many owned by Rupert Murdoch's News Corp - consider forcing people to pay for content.
Mr Murdoch has also said he may block Google from indexing and displaying news content from his companies.