Arron Banks, the biggest individual donor in British political history and a major source of money behind the Brexit campaign, has been placed under criminal investigation for several suspected offenses that took place during the referendum.

Britain’s election watchdog says there are “reasonable grounds” to suspect that Banks committed several crimes in the run-up to the dramatic vote, and that they suspect he wasn’t the true source of £8 million ($10 million) in loans made to Better for the Country—a company he used to finance the Leave.EU campaign group whose public face was Nigel Farage.

The investigation by the National Crime Agency, which has the expertise to trace illicit cross-border money trails, will seek to find the true source of the money that funded Brexit.

Banks—one of the self-christened “bad boys of Brexit” who met Donald Trump in late 2016 with Farage—has long been a controversial figure with business links to Russia. He is known to have been offered three Russian business deals during the Brexit campaign, including one that gave him the chance to make huge profits from a Russian gold company.

Banks is also known to have met Alexander Yakovenko, Vladimir Putin’s U.K. envoy, on at least four occasions. The meetings and offers have been interpreted by some as attempts by Moscow to influence Brexit.

In its statement Thursday, Britain’s Electoral Commission watchdog said it suspected the huge donation to the Leave.EU campaign came not from Banks but “from impermissible sources.” Banks denies receiving money from Russia or any other foreign country, according to the BBC.

“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. The Electoral Commission has produced no evidence to the contrary.”

The other Brexit campaign, Vote Leave, has already been found to have broken electoral law for exceeding spending limits. But Banks and Leave.EU have been referred to Britain’s National Crime Agency, suggesting the suspected offenses are more serious and outside the limits of electoral law.

The National Crime Agency said in a statement: “While electoral law offenses would not routinely fall within the NCA’s remit, the nature of the necessary inquiries and the potential for offenses to have been committed other than under electoral law lead us to consider an NCA investigation appropriate in this instance.”

The Labour party’s Ian Murray, a pro-EU member of parliament, told The Daily Beast the fresh allegations cast doubt on the result of the referendum and help make the case for a second vote—but time is running out before Britain’s scheduled exit from the EU in March next year.

“The Leave campaign have already been found to have broken electoral law so I don’t think anyone would be surprised by these accusations,” said Murray. “All these accusations and conclusions on the Leave campaign highlight more than ever why we need a vote on any Brexit deal.”

Andy Wigmore, a spokesperson for Leave.EU, dismissed the accusations, saying: “It’s completely to be expected. It will finally bring a head to all these crazy allegations made about us. We’re not worried.”