The U.K. data commissioner has fined a campaign backing Britain's departure from the European Union and an insurance company founded by its millionaire backer a total of 135,000 pounds ($176,000) for breaches of data laws.
"We are investigating allegations that Eldon Insurance Services Limited shared customer data obtained for insurance purposes with Leave.EU", it said.
In a major report published by the UK's Information Commissioner on Tuesday, Elizabeth Denham confirmed that a Leave.EU newsletter was sent to more than 319,000 email addresses on Banks' Eldon Insurance company database in the run up to the referendum.
More than a million emails were said to have been sent between February and July 2017 which included the company's banner and a discount offer for Leave.EU supporters.
The ICO also intends to fine Leave.EU a further £15,000 for a "separate, serious breach" after nearly 300,000 emails were sent to Eldon Insurance customers containing a Leave.EU newsletter.
The Electoral Commission announced last week there were reasonable grounds to suspect that Mr Banks was not the true source of £8m in loans made to the Better for the Country organisation, which ran his group Leave.EU.
But it did not conclude that data harvesting techniques had been a decisive factor in the Brexit vote, stating that "it is impossible for us to say whether the data techniques used by either side in the UK EU referendum campaign impacted on the result".
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Leave.EU was the biggest unofficial campaign.
Denham's deputy, James Dipple-Johnstone, said the watchdog also had concerns about the use of data for electronic marketing at Vote Leave, the official pro-Brexit campaign group led by Boris Johnson and Michael Gove.
Since then, Banks has faced questions about his tactics and finances in the referendum.
"We need to look at whether the processes are working to be able to separate the data from political campaigning and from insurance use", she said.
Meanwhile, Denham has called on the government to offer the United Kingdom a data privacy regime "fit for goal in the digital age, warning that voluntary moves by online company are not enough to solve the problem".
"The sharing of information has gone both ways" at Banks' businesses, she added, and "the fines could be significantly higher" if further "misdeeds" were uncovered.