LONDON – Britain’s National Crime Agency is investigating a main financial backer of the campaign to get Britain out of the European Union over suspected illegal funding during the country’s EU membership referendum, authorities said Thursday.

The Electoral Commission said wealthy British businessman Arron Banks, his group Leave.EU “and other associated companies and individuals” are under criminal investigation. The inquiry concerns 8 million pounds reportedly loaned by Banks and his companies to a pro-Brexit group, Better for the Country.

The commission said it suspects Banks “was not the true source” of the money and concealed its real origins.

“The financial transactions we have investigated include companies incorporated in Gibraltar and the Isle of Man,” said Bob Posner, the electoral watchdog’s director of political finance.

“Our investigation has unveiled evidence that suggests criminal offences have been committed which fall beyond the remit of the commission,” he added. “This is why we have handed our evidence to the NCA to allow them to investigate and take any appropriate law-enforcement action.”

The crime agency confirmed the investigation but said it could not discuss “any operational detail.”

British political parties and political groups are barred from taking money from overseas-based individuals or businesses during electoral campaigns.

Banks, a brash multimillionaire insurance executive, said he was confident that a “full and frank investigation” would clear him.

“There is no evidence of any wrongdoing from the companies I own,” he said. “I am a U.K. taxpayer and I have never received any foreign donations.”

Ever since Britain voted by a margin of 52 percent to 48 percent in 2016 to leave the 28-nation EU, opponents of Brexit have raised questions about the source of funding for the “leave” campaign, possible Russian influence on the vote and the role of social-media advertising using data harvested from millions of Facebook users by the firm Cambridge Analytica.

A British parliamentary committee is investigating Banks’ role in the referendum and his meetings with Russian officials as part of a wide-ranging inquiry into disinformation and “fake news.”

Earlier this year the committee criticized Banks for seeming to “want to hide the extent of his contacts with Russia.”

Banks, a close ally of former U.K. Independence Party leader Nigel Farage and an enthusiastic U.K. supporter of President Donald Trump, denies any wrongdoing. He has accused pro-EU politicians and activists of “trying to discredit the Brexit campaign.”

“I’d like to think I’m an evil genius with a white cat that kind of controls the whole of Western democracy, but clearly that’s nonsense,” Banks told the parliamentary committee in June.