The largest US newspaper publishers on Thursday threatened legal action to block a California startup's plan for a dedicated "news browser" with its own advertising.

The companies joined in a letter to the startup called Brave, which announced its plan earlier this year for its interface which blocks "harmful" advertising and replaces it with "safer" messages.

The newspaper groups said in the letter that this plan "violates the law," and that the companies "intend to fully enforce their rights."

"Your plan to use our content to sell your advertising is indistinguishable from a plan to steal our content to publish on your own website," said the letter, which was endorsed by the New York Times Co., Washington Post, Dow Jones, Tribune Publishing, Digital First Media, McClatchy and 11 other publishers.

"Your public statements demonstrate clearly that you intend to harness and exploit the content of all the publishers on the Web to sell your own advertising."

The plan unveiled in January by Brave, started by Mozilla co-founder Brendan Eich and software engineer Marshall Rose, claimed it would "fix the Web" by allowing users to circumvent advertising which tracks user browsing and can sometimes be infected with malware.

The Brave browser would strip out the ads on news websites, and replace them with different ads, sharing revenue with the publishers. A public version of the browser was set to be released this week.

"The new Brave browser automatically blocks ads and trackers, making it faster and safer than your current browser," the company says on its website.

The publishers, who operate the largest dailies in the United States, said they would not participate in this effort despite Brave's offer of revenue sharing.

"Your apparent plan to permit your customers to make Bitcoin 'donations' to us, and for you to donate to us some unspecified percentage of revenue you receive from the sale of your ads on our sites, cannot begin to compensate us for the loss of our ability to fund our work by displaying our own advertising," said the letter from the 17 company attorneys.

"We expressly decline to participate in any way in Brave's supposed business model ... and we refuse to accept any 'site wallet' that you propose to create for our supposed benefit."

Brave said in a statement that the letter "is filled with false assertions and that the newspapers have "fundamentally misunderstood Brave."

"Brave is the solution, not the enemy," the statement emailed to AFP said.

Brave said the claim that it was "republishing" the content was inaccurate because it was simply a browser like many others which "can block, rearrange, mash-up and otherwise make use of any content from any source."

The statement said the letter "is the first shot in a war on all ad-blockers, not just on Brave" and added that "we would be happy to sit down" with publishers "for an opportunity to discuss how the Brave solution can be a win-win."