LONDON — Arron Banks, a British insurance mogul, was referred to Britain’s equivalent of the FBI on Thursday after an investigation into Brexit campaign funding concluded that payments he purportedly made to pro-Brexit groups came from “impermissible” sources.

Banks now faces a criminal investigation by the National Crime Agency (NCA) over suspected illegal campaign funding during the 2016 referendum in which Britain voted 52 percent to 48 percent to leave the European Union.

One of the “Bad Boys of Brexit” who have come under scrutiny for ties to Russia and the Trump presidential campaign, Banks was famously pictured with President Trump and the Euroskeptic British politician Nigel Farage at Trump Tower soon after Trump was elected. In Britain, parliamentary investigators have probed his dealings with the Russian ambassador in London, which Banks has described as routine business exchanges.

Britain’s Electoral Commission, which spent the past year looking into Brexit campaign funding, said in a statement Thursday it has “reasonable grounds to suspect” that Banks was not the “true source” of the $10 million in loans given to Leave.EU, the pro-Brexit group headed by Farage, and its parent company, Better for the Country.

Noting that “various criminal offences may have been committed,” the watchdog group said it was referring the matter to the NCA, which has broader powers, including the authority to pursue international leads. It normally investigates serious and organized crime.

The NCA said in a separate statement that it would be investigating possible offenses beyond violations of electoral law.

Banks, whose pro-Brexit contributions made him the largest political donor in British history, said he welcomed the NCA’s involvement.

“I am confident that a full and frank investigation will finally put an end to the ludicrous allegations levelled against me and my colleagues,” he said in a statement. “. . . I am a UK taxpayer and I have never received any foreign donations.”

Banks, a colorful figure and combative force on social media, also tweeted a smiling selfie of himself in Bermuda, where he said he is fishing.

Elizabeth Bilney, chairwoman of the Leave.EU campaign, was also referred to the NCA. She told the BBC she did nothing wrong and predicted that she and Banks would be exonerated.

Asked whether she could substantiate Banks’s claim that none of the money came from Russia, she said: “I can confirm it wouldn’t have come from Russia. I run the group of companies where the money was from, and we don’t have any transactions that are from Russia.”

Pro-Europeans said the new investigation bolstered the case for a second Brexit referendum.

“This is a significant,” tweeted Liberal Democrat lawmaker Wera Hobhouse. “Where did the money spent on the #leave campaign come from? Even more urgent now to get @peoplesvote_uk

Other opposition lawmakers said Brexit should be postponed.

“Brexit must be put on hold until we know the extent of these crimes against our democracy,” tweeted Labour lawmaker David Lammy.

The Electoral Commission said that some of the loans to the Brexit campaign groups were from Banks’s company Rock Holdings, incorporated in the Isle of Man, a self-governing dependency of the crown and therefore an “impermissible” source under Britain’s electoral law.

British political campaigns are barred from taking money from companies and individuals based outside the country during specific time frames. Banks has drawn criticism in the past for his links to firms in Gibraltar, as well as the Isle of Man, places that offer ultralow taxes.

Damian Collins, who chairs a parliamentary committee that has previously questioned Banks — the businessman made headlines when he walked out of the session early — said his committee would question the Electoral Commission next week about the new probe.

“These are serious issues that should be fully investigated by the NCA using their all powers,” he said.

For his part, Banks claimed he was being investigated because of “intense political pressure from anti-Brexit supporters.”