The Electoral Commission said Leave.EU, which was allowed to spend 700,000 pounds ($950,000), overspent by at least 77,380 pounds and presented "incomplete and inaccurate" information about its spending.
The commission said it was fining the group 70,000 pounds and referring CEO Liz Bilney to the Metropolitan Police. "Leave.EU exceeded its spending limit and failed to declare its funding and its spending correctly," said the commission's director of regulation, Bob Posner. "These are serious offenses."
Leave.EU founder Arron Banks accused the commission of "a politically motivated attack on Brexit and the 17.4 million people who defied the establishment to vote for an independent Britain." Several inquiries are underway into spending and advertising during the referendum, which was won 52 percent to 48 percent by the "leave" side.
Some inquiries focus on the role of online advertising amid allegations about the use of Facebook users' data to influence U.S. and British campaigns. The Electoral Commission said it had found no evidence that Leave.EU received donations or paid services from Cambridge Analytica, the data firm at the center of the Facebook privacy scandal.