Internet users in Britain could be automatically warned by their broadband provider if they are visiting an untrustworthy news website as part of a deal involving the news ranking service NewsGuard.

Every major UK news website has been scored for transparency and accuracy by the private company, which launched in Britain on Wednesday. It is seeking to establish itself as the de facto authority on the trustworthiness of news sites. It ranks publishers on nine criteria dealing with credibility and transparency, ranging from whether they disclose details of their ownership to whether they regularly publish false stories, producing a lengthy “nutrition label” to judge outlets on their integrity.

Its co-founders Steve Brill and Gordon Crovitz told the Guardian they were in talks with major British broadband providers to build NewsGuard into their services. Under this plan individuals attempting to access a news site deemed to be untrustworthy by the company would be met with a warning page served by their internet service provider, with customers able to turn the setting on or off in a similar manner to blocks on pornography websites.

The service, already available as a plug-in for most popular browsers, has been live in the US since last summer but this is its first overseas launch. NewsGuard has hired editors, overseen by the former BBC News channel boss Richard Sambrook, to judge about 150 major British news outlets, ranking them on nine criteria and writing lengthy labels on whether they can be trusted.

Brill suggested the site was more effective than fact-checking organisations because it warned users about an entire outlet. “We can act as a megaphone for what a fact-checking organisation has said because we’re looking at the whole site,” he said.

It appears almost all mainstream UK news outlets achieved a trustworthy ranking, ranging from the Sun to the Daily Star. But not all of the news sites received a clean bill of health, even if they were considered to be trustworthy. The Daily Express website received an overall approval rating but was flagged for failing to “gather and present information responsibly” and failing to “avoid deceptive headlines”, albeit while noting the outlet’s “irresponsible coverage of issues related to Muslims has abated in the last year”.

The Independent, which was listed as having failed to disclose its ownership and financing, scored seven out of nine, while the pro-Jeremy Corbyn site the Canary received eight out of nine. The Guardian scored full marks on all measures.

After the Guardian highlighted an untrustworthy rating given to MailOnline, NewsGuard faced a barrage of attacks from the online news outlet. It re-evaluated its judgment and upgraded MailOnline’s ranking to trustworthy.

NewsGuard hopes to capitalise on a push from governments and technology companies around the world to find ways to combat online disinformation. The company’s founders argue that human editors are the answer to the problem rather than further investment in algorithms, with launches imminent in Germany, France, and Italy.

The service has also received the endorsement of the British librarian association Ascel, which is encouraging school and public libraries to install the plug-in on their computers. It already comes pre-installed on the mobile version of Microsoft’s Edge browser.

It has come under attack from the owners of some sites unhappy with their negative ratings, including the Kremlin-backed RT. Breitbart News has been vociferous in criticising the company for having a supposed bias towards “corporate media”, although the founders insist they are transparent and potential tech clients have not detected any bias in their judgments.

On Wednesday the company announced that the former Nato chief Anders Fogh Rasmussen and the Wikipedia founder, Jimmy Wales, would be joining its advisory board.

NewsGuard also actively encourages news websites to game its system, arguing that it provides an incentive for sites to improve their transparency. It cited Al-Jazeera English’s decision to add a disclaimer that it is funded by the Qatari government in order to improve its rating.