Britain's Electoral Commission says it is investigating whether the influential pro-Brexit Leave.EU campaign group broke the law over donations it received during last year's European Union referendum.
"The investigation is focused on whether one or more donations - including of services - accepted by Leave.EU was impermissible; and whether Leave.EU's spending return was complete," the electoral watchdog said in a statement on Friday.
Leave.EU chairman Arron Banks denied any wrongdoing.
The findings of the investigation could not put the result of the referendum itself in question.
The group did not lead the official campaign for Britain to quit the EU, but was credited with mobilising people who do not typically vote and helping influence regular voters. Britain voted 52 to 48 percent in favour of 'leave'.
The commission has the power to impose fines and other sanctions if it find rules were broken, and in some cases can refer matters to the police for criminal investigation.
"We fully and correctly complied with the Electoral Commission rules on spending during the referendum. We have answered all of their questions," Banks said in a statement.
He criticised the timing of the announcement, which comes as Britain prepares for a June 8 election, as politically motivated.
"We will not be cooperating any further with the commission and we will see them in court," he said.