A billionaire accused by British politicians of being a suspected Russian agent has turned to the American courts in an attempt to “vindicate his reputation”, BuzzFeed News can reveal.

Christopher Chandler, the 58-year-old New Zealand–born founder of a think tank that had a significant influence on the Brexit debate, claims that a private investigator based in Washington, DC, was the original source of “demonstrably false” allegations of money laundering, organised crime, and Russian espionage against him and his brother, Richard, that caused a sensation in London earlier this year.

Chandler, a financier based in Dubai, is suing Donald Berlin and his company, Investigative Consultants, Inc., for $15 million (about £11.6 million) in damages. The libel case was filed in the US District Court for the District of Columbia last month and is reported here for the first time.

“Mr Chandler and his brother have never been spies or engaged in espionage for any country, let alone Russia,” the lawsuit states.

The US legal action adds another extraordinary twist to a story that has gripped British politics.

Chandler became a figure of intrigue in late 2017 after his think tank, the Legatum Institute, became closely associated with Brexit-supporting cabinet ministers. Curiosity turned to alarm when word spread that Chandler had allegedly worked for Russian intelligence. At a time of heightened anxiety about Russian aggression in the West, the possibility that a Kremlin ally had penetrated the top levels of British politics struck many in Westminster as an urgent threat to national security.

Chandler adamantly denied the claims, which he says have caused huge damage to his reputation and business. In launching the US lawsuit, his lawyers have put documents setting out these claims in detail into the public domain for the first time.

The proceedings against Berlin will expose Chandler’s background and business activities to a degree of scrutiny that the billionaire has spent his career avoiding.

The lawsuit claims that Berlin, while portraying himself as an experienced investigator with good sources, defrauded customers by providing worthless background reports that he put together by “inventing salacious narratives from his own imagination and weaving those false narratives together with manipulated information from the public domain”.

According to the lawsuit, Berlin was approached by Prince Albert of Monaco’s intelligence adviser to conduct a background check on the Chandlers in 2002, when the brothers lived in Monte Carlo. The suit claims Berlin tried to defraud the prince by producing a fabricated report. It allegedly included details lifted word-for-word from old news articles Berlin found on the internet, while other details, the suit claims, were inspired by “outdated spy novels”.

Berlin is contesting the claim. According to his lawyers, he is a small-business owner with a high-level US government security clearance who simply carried out a background check as requested by a client 15 years ago. They say Chandler has tried to intimidate him into providing a false retraction, but that he refused to go along with it.