Nigel Farage could face punishment from the elections watchdog after failing to declare £200,000-worth of donations over more than a decade.
The Ukip leader apparently disclosed to Brussels that he had been receiving free use of an office in Britain since 2001, but did not tell the Electoral Commission.
The issue is said to have emerged only when officials saw the barn mentioned in news stories in the run-up to recent polls.
"When we became aware of the potential unreported donations, we were in correspondence with Mr Farage," a spokeswoman for the commission said.
"We have not yet made a decision about whether any further action will be taken against Mr Farage for reporting the donations late."