Fake news websites registered in the UK and made to resemble trusted British outlets are allegedly spreading disinformation about western companies operating in Ukraine.

The suspected Russian propaganda operation has prompted calls by parliamentarians for a change in the law to force UK-registered news websites to reveal their ownership, as happens in the EU.

While the sites – londoninsider.co.uk and talk-finance.co.uk – are in English and have been registered in the UK, their output has been picked up and disseminated in Ukraine, where the UK’s media has a reputation for reliability and trustworthiness.

The use of the sites has been highlighted by a US firm, Sarn, which is working in Ukraine in the energy and military hardware sectors. It said articles on the two sites had falsely accused it of arms trafficking, judicial fraud and embezzlement.

Content on the sites appears to be AI-generated, while an analysis by a linguist engaged by Sarn suggested that the original text had been created by a Russian speaker.

The name of an actual UK journalist appeared as a byline on the stories, but he has said in an affidavit that he did not know anything about the story he was supposed to have written for londoninsider.co.uk.

A Liberal Democrat peer, Lord Wallace of Saltaire, said: “It is extremely concerning that London’s reputation as a media and financial hub is being used by the Russian propaganda machine to damage western companies operating in Ukraine.”

Lord Wallace added that transparency of ownership to enable the public to know who controls the media was more important than ever before and said all news outlets, regardless of their size, should have to publish information about their owners in a national database.

“This important move to protect the media from political or economic interference was brought into law across the European Union in March and it’s high time it was introduced here.”

Armen Agas, Sarn’s deputy chair, said the company categorically rejected “baseless and entirely fake news” that it said was “propagated” by the sites under the headline: Weapons, Money and Sarn: How the Czech-American Group Embezzled Ukraine.

“We believe we’ve been targeted because of our economic development work in Ukraine during the conflict,” they added.

Sarn said it had engaged cybersecurity experts and lawyers and that one of the sites – talk-finance.co.uk – responded to a legal “cease-and-desist letter” by demanding money. The other site continues to publish the article.

A request for comment was sent by the Guardian to the only contact details provided by talk-finance.co.uk, a Hotmail address. There was a swift reply from an unnamed person, who said they were ready to meet in Calais’ “tent city”, the name given to an area where asylum seekers and immigrants planning to cross the channel to the UK have been living.

A request for comment was also sent to an email address on the londoninsider.co.uk website, where the site is described as “a leading digital magazine covering the latest political, business, sport and showbiz news”.

There was a response from a person named Thomas Henwell, who said the site published “fact-based news” and suggested Sarn was attempting to intimidate it. They insisted it had published many other stories that were “not positive” for the Russian government.

But they declined to say who owned the site and, in a further email, described it as a “private owned enterprise” relying on advertising revenue. They declined to speak on the phone or by Zoom.

The cases of the fake UK news websites come amid an explosion in online disinformation campaigns from Russia, before and after its invasion of Ukraine.

NewsGuard, a company that aims to counter misinformation by studying and rating news websites, has found that false narratives about Ukraine and its allies were already proliferating online before the February 2022 invasion.

The company said it had debunked more than 250 false narratives related to the Russia-Ukraine war, and identified 627 sites spreading those myths. The false narratives have ranged from propaganda claims that reports of massacres by Russian forces in locations such as Bucha were “staged” to assertions that Nazi ideology is driving Ukraine’s political leadership.