The Leave.EU campaign group is facing an investigation into "potential offences under the law" over its European Union referendum spending returns.
The Electoral Commission said the move followed an assessment that there were "reasonable grounds" to suspect the law had been broken.
In a statement, the official elections watchdog said the inquiry would focus on whether the campaign group had received "impermissible" donations and whether its spending return was incomplete.
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"The Electoral Commission has begun an investigation into Leave.EU's EU Referendum spending return," the statement said.
"This followed an assessment which concluded that there were reasonable grounds to suspect that potential offences under the law may have occurred.
"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.
"Once the investigation is complete, the commission will decide whether any breaches have occurred and, if so, what further action may be appropriate, in line with its enforcement policy."